xref: /dports/math/py-matplotlib/matplotlib-3.4.3/tutorials/intermediate/constrainedlayout_guide.py

"""
================================
Constrained Layout Guide
================================

How to use constrained-layout to fit plots within your figure cleanly.

*constrained_layout* automatically adjusts subplots and decorations like
legends and colorbars so that they fit in the figure window while still
preserving, as best they can, the logical layout requested by the user.

*constrained_layout* is similar to
:doc:`tight_layout</tutorials/intermediate/tight_layout_guide>`,
but uses a constraint solver to determine the size of axes that allows
them to fit.

*constrained_layout* needs to be activated before any axes are added to
a figure. Two ways of doing so are

* using the respective argument to :func:`~.pyplot.subplots` or
  :func:`~.pyplot.figure`, e.g.::

      plt.subplots(constrained_layout=True)

* activate it via :ref:`rcParams<matplotlib-rcparams>`, like::

      plt.rcParams['figure.constrained_layout.use'] = True

Those are described in detail throughout the following sections.

.. warning::

    Currently Constrained Layout is **experimental**.  The
    behaviour and API are subject to change, or the whole functionality
    may be removed without a deprecation period.  If you *require* your
    plots to be absolutely reproducible, get the Axes positions after
    running Constrained Layout and use ``ax.set_position()`` in your code
    with ``constrained_layout=False``.

Simple Example
==============

In Matplotlib, the location of axes (including subplots) are specified in
normalized figure coordinates. It can happen that your axis labels or
titles (or sometimes even ticklabels) go outside the figure area, and are thus
clipped.
"""

# sphinx_gallery_thumbnail_number = 18


import matplotlib.pyplot as plt
import matplotlib.colors as mcolors
import matplotlib.gridspec as gridspec
import numpy as np

plt.rcParams['savefig.facecolor'] = "0.8"
plt.rcParams['figure.figsize'] = 4.5, 4.
plt.rcParams['figure.max_open_warning'] = 50


def example_plot(ax, fontsize=12, hide_labels=False):
    ax.plot([1, 2])

    ax.locator_params(nbins=3)
    if hide_labels:
        ax.set_xticklabels([])
        ax.set_yticklabels([])
    else:
        ax.set_xlabel('x-label', fontsize=fontsize)
        ax.set_ylabel('y-label', fontsize=fontsize)
        ax.set_title('Title', fontsize=fontsize)


fig, ax = plt.subplots(constrained_layout=False)
example_plot(ax, fontsize=24)

###############################################################################
# To prevent this, the location of axes needs to be adjusted. For
# subplots, this can be done by adjusting the subplot params
# (:ref:`howto-subplots-adjust`). However, specifying your figure with the
# ``constrained_layout=True`` kwarg will do the adjusting automatically.

fig, ax = plt.subplots(constrained_layout=True)
example_plot(ax, fontsize=24)

###############################################################################
# When you have multiple subplots, often you see labels of different
# axes overlapping each other.

fig, axs = plt.subplots(2, 2, constrained_layout=False)
for ax in axs.flat:
    example_plot(ax)

###############################################################################
# Specifying ``constrained_layout=True`` in the call to ``plt.subplots``
# causes the layout to be properly constrained.

fig, axs = plt.subplots(2, 2, constrained_layout=True)
for ax in axs.flat:
    example_plot(ax)

###############################################################################
# Colorbars
# =========
#
# If you create a colorbar with `.Figure.colorbar`,
# you need to make room for it.  ``constrained_layout`` does this
# automatically.  Note that if you specify ``use_gridspec=True`` it will be
# ignored because this option is made for improving the layout via
# ``tight_layout``.
#
# .. note::
#
#   For the `~.axes.Axes.pcolormesh` kwargs (``pc_kwargs``) we use a
#   dictionary. Below we will assign one colorbar to a number of axes each
#   containing a `~.cm.ScalarMappable`; specifying the norm and colormap
#   ensures the colorbar is accurate for all the axes.

arr = np.arange(100).reshape((10, 10))
norm = mcolors.Normalize(vmin=0., vmax=100.)
# see note above: this makes all pcolormesh calls consistent:
pc_kwargs = {'rasterized': True, 'cmap': 'viridis', 'norm': norm}
fig, ax = plt.subplots(figsize=(4, 4), constrained_layout=True)
im = ax.pcolormesh(arr, **pc_kwargs)
fig.colorbar(im, ax=ax, shrink=0.6)

############################################################################
# If you specify a list of axes (or other iterable container) to the
# ``ax`` argument of ``colorbar``, constrained_layout will take space from
# the specified axes.

fig, axs = plt.subplots(2, 2, figsize=(4, 4), constrained_layout=True)
for ax in axs.flat:
    im = ax.pcolormesh(arr, **pc_kwargs)
fig.colorbar(im, ax=axs, shrink=0.6)

############################################################################
# If you specify a list of axes from inside a grid of axes, the colorbar
# will steal space appropriately, and leave a gap, but all subplots will
# still be the same size.

fig, axs = plt.subplots(3, 3, figsize=(4, 4), constrained_layout=True)
for ax in axs.flat:
    im = ax.pcolormesh(arr, **pc_kwargs)
fig.colorbar(im, ax=axs[1:, ][:, 1], shrink=0.8)
fig.colorbar(im, ax=axs[:, -1], shrink=0.6)

####################################################
# Suptitle
# =========
#
# ``constrained_layout`` can also make room for `~.figure.Figure.suptitle`.

fig, axs = plt.subplots(2, 2, figsize=(4, 4), constrained_layout=True)
for ax in axs.flat:
    im = ax.pcolormesh(arr, **pc_kwargs)
fig.colorbar(im, ax=axs, shrink=0.6)
fig.suptitle('Big Suptitle')

####################################################
# Legends
# =======
#
# Legends can be placed outside of their parent axis.
# Constrained-layout is designed to handle this for :meth:`.Axes.legend`.
# However, constrained-layout does *not* handle legends being created via
# :meth:`.Figure.legend` (yet).

fig, ax = plt.subplots(constrained_layout=True)
ax.plot(np.arange(10), label='This is a plot')
ax.legend(loc='center left', bbox_to_anchor=(0.8, 0.5))

#############################################
# However, this will steal space from a subplot layout:

fig, axs = plt.subplots(1, 2, figsize=(4, 2), constrained_layout=True)
axs[0].plot(np.arange(10))
axs[1].plot(np.arange(10), label='This is a plot')
axs[1].legend(loc='center left', bbox_to_anchor=(0.8, 0.5))

#############################################
# In order for a legend or other artist to *not* steal space
# from the subplot layout, we can ``leg.set_in_layout(False)``.
# Of course this can mean the legend ends up
# cropped, but can be useful if the plot is subsequently called
# with ``fig.savefig('outname.png', bbox_inches='tight')``.  Note,
# however, that the legend's ``get_in_layout`` status will have to be
# toggled again to make the saved file work, and we must manually
# trigger a draw if we want constrained_layout to adjust the size
# of the axes before printing.

fig, axs = plt.subplots(1, 2, figsize=(4, 2), constrained_layout=True)

axs[0].plot(np.arange(10))
axs[1].plot(np.arange(10), label='This is a plot')
leg = axs[1].legend(loc='center left', bbox_to_anchor=(0.8, 0.5))
leg.set_in_layout(False)
# trigger a draw so that constrained_layout is executed once
# before we turn it off when printing....
fig.canvas.draw()
# we want the legend included in the bbox_inches='tight' calcs.
leg.set_in_layout(True)
# we don't want the layout to change at this point.
fig.set_constrained_layout(False)
fig.savefig('../../doc/_static/constrained_layout_1b.png',
            bbox_inches='tight', dpi=100)

#############################################
# The saved file looks like:
#
# .. image:: /_static/constrained_layout_1b.png
#    :align: center
#
# A better way to get around this awkwardness is to simply
# use the legend method provided by `.Figure.legend`:
fig, axs = plt.subplots(1, 2, figsize=(4, 2), constrained_layout=True)
axs[0].plot(np.arange(10))
lines = axs[1].plot(np.arange(10), label='This is a plot')
labels = [l.get_label() for l in lines]
leg = fig.legend(lines, labels, loc='center left',
                 bbox_to_anchor=(0.8, 0.5), bbox_transform=axs[1].transAxes)
fig.savefig('../../doc/_static/constrained_layout_2b.png',
            bbox_inches='tight', dpi=100)

#############################################
# The saved file looks like:
#
# .. image:: /_static/constrained_layout_2b.png
#    :align: center
#

###############################################################################
# Padding and Spacing
# ===================
#
# Padding between axes is controlled in the horizontal by *w_pad* and
# *wspace*, and vertical by *h_pad* and *hspace*.  These can be edited
# via `~.figure.Figure.set_constrained_layout_pads`.  *w/h_pad* are
# the minimum space around the axes in units of inches:

fig, axs = plt.subplots(2, 2, constrained_layout=True)
for ax in axs.flat:
    example_plot(ax, hide_labels=True)
fig.set_constrained_layout_pads(w_pad=4 / 72, h_pad=4 / 72, hspace=0, wspace=0)

##########################################
# Spacing between subplots is further set by *wspace* and *hspace*. These
# are specified as a fraction of the size of the subplot group as a whole.
# If these values are smaller than *w_pad* or *h_pad*, then the fixed pads are
# used instead. Note in the below how the space at the edges doesn't change
# from the above, but the space between subplots does.

fig, axs = plt.subplots(2, 2, constrained_layout=True)
for ax in axs.flat:
    example_plot(ax, hide_labels=True)
fig.set_constrained_layout_pads(w_pad=4 / 72, h_pad=4 / 72, hspace=0.2,
                                wspace=0.2)

##########################################
# If there are more than two columns, the *wspace* is shared between them,
# so here the wspace is divided in 2, with a *wspace* of 0.1 between each
# column:

fig, axs = plt.subplots(2, 3, constrained_layout=True)
for ax in axs.flat:
    example_plot(ax, hide_labels=True)
fig.set_constrained_layout_pads(w_pad=4 / 72, h_pad=4 / 72, hspace=0.2,
                                wspace=0.2)

##########################################
# GridSpecs also have optional *hspace* and *wspace* keyword arguments,
# that will be used instead of the pads set by ``constrained_layout``:

fig, axs = plt.subplots(2, 2, constrained_layout=True,
                        gridspec_kw={'wspace': 0.3, 'hspace': 0.2})
for ax in axs.flat:
    example_plot(ax, hide_labels=True)
# this has no effect because the space set in the gridspec trumps the
# space set in constrained_layout.
fig.set_constrained_layout_pads(w_pad=4 / 72, h_pad=4 / 72, hspace=0.0,
                                wspace=0.0)
plt.show()

##########################################
# Spacing with colorbars
# -----------------------
#
# Colorbars are placed a distance *pad* from their parent, where *pad*
# is a fraction of the width of the parent(s).  The spacing to the
# next subplot is then given by *w/hspace*.

fig, axs = plt.subplots(2, 2, constrained_layout=True)
pads = [0, 0.05, 0.1, 0.2]
for pad, ax in zip(pads, axs.flat):
    pc = ax.pcolormesh(arr, **pc_kwargs)
    fig.colorbar(pc, ax=ax, shrink=0.6, pad=pad)
    ax.set_xticklabels('')
    ax.set_yticklabels('')
    ax.set_title(f'pad: {pad}')
fig.set_constrained_layout_pads(w_pad=2 / 72, h_pad=2 / 72, hspace=0.2,
                                wspace=0.2)

##########################################
# rcParams
# ========
#
# There are five :ref:`rcParams<matplotlib-rcparams>` that can be set,
# either in a script or in the :file:`matplotlibrc` file.
# They all have the prefix ``figure.constrained_layout``:
#
# - *use*: Whether to use constrained_layout. Default is False
# - *w_pad*, *h_pad*:    Padding around axes objects.
#   Float representing inches.  Default is 3./72. inches (3 pts)
# - *wspace*, *hspace*:  Space between subplot groups.
#   Float representing a fraction of the subplot widths being separated.
#   Default is 0.02.

plt.rcParams['figure.constrained_layout.use'] = True
fig, axs = plt.subplots(2, 2, figsize=(3, 3))
for ax in axs.flat:
    example_plot(ax)

#############################
# Use with GridSpec
# =================
#
# constrained_layout is meant to be used
# with :func:`~matplotlib.figure.Figure.subplots` or
# :func:`~matplotlib.gridspec.GridSpec` and
# :func:`~matplotlib.figure.Figure.add_subplot`.
#
# Note that in what follows ``constrained_layout=True``

fig = plt.figure()

gs1 = gridspec.GridSpec(2, 1, figure=fig)
ax1 = fig.add_subplot(gs1[0])
ax2 = fig.add_subplot(gs1[1])

example_plot(ax1)
example_plot(ax2)

###############################################################################
# More complicated gridspec layouts are possible.  Note here we use the
# convenience functions `~.Figure.add_gridspec` and
# `~.SubplotSpec.subgridspec`.

fig = plt.figure()

gs0 = fig.add_gridspec(1, 2)

gs1 = gs0[0].subgridspec(2, 1)
ax1 = fig.add_subplot(gs1[0])
ax2 = fig.add_subplot(gs1[1])

example_plot(ax1)
example_plot(ax2)

gs2 = gs0[1].subgridspec(3, 1)

for ss in gs2:
    ax = fig.add_subplot(ss)
    example_plot(ax)
    ax.set_title("")
    ax.set_xlabel("")

ax.set_xlabel("x-label", fontsize=12)

############################################################################
# Note that in the above the left and right columns don't have the same
# vertical extent.  If we want the top and bottom of the two grids to line up
# then they need to be in the same gridspec.  We need to make this figure
# larger as well in order for the axes not to collapse to zero height:

fig = plt.figure(figsize=(4, 6))

gs0 = fig.add_gridspec(6, 2)

ax1 = fig.add_subplot(gs0[:3, 0])
ax2 = fig.add_subplot(gs0[3:, 0])

example_plot(ax1)
example_plot(ax2)

ax = fig.add_subplot(gs0[0:2, 1])
example_plot(ax, hide_labels=True)
ax = fig.add_subplot(gs0[2:4, 1])
example_plot(ax, hide_labels=True)
ax = fig.add_subplot(gs0[4:, 1])
example_plot(ax, hide_labels=True)
fig.suptitle('Overlapping Gridspecs')


############################################################################
# This example uses two gridspecs to have the colorbar only pertain to
# one set of pcolors.  Note how the left column is wider than the
# two right-hand columns because of this.  Of course, if you wanted the
# subplots to be the same size you only needed one gridspec.


def docomplicated(suptitle=None):
    fig = plt.figure()
    gs0 = fig.add_gridspec(1, 2, figure=fig, width_ratios=[1., 2.])
    gsl = gs0[0].subgridspec(2, 1)
    gsr = gs0[1].subgridspec(2, 2)

    for gs in gsl:
        ax = fig.add_subplot(gs)
        example_plot(ax)
    axs = []
    for gs in gsr:
        ax = fig.add_subplot(gs)
        pcm = ax.pcolormesh(arr, **pc_kwargs)
        ax.set_xlabel('x-label')
        ax.set_ylabel('y-label')
        ax.set_title('title')

        axs += [ax]
    fig.colorbar(pcm, ax=axs)
    if suptitle is not None:
        fig.suptitle(suptitle)


docomplicated()

###############################################################################
# Manually setting axes positions
# ================================
#
# There can be good reasons to manually set an axes position.  A manual call
# to `~.axes.Axes.set_position` will set the axes so constrained_layout has
# no effect on it anymore. (Note that ``constrained_layout`` still leaves the
# space for the axes that is moved).

fig, axs = plt.subplots(1, 2)
example_plot(axs[0], fontsize=12)
axs[1].set_position([0.2, 0.2, 0.4, 0.4])

###############################################################################
# Manually turning off ``constrained_layout``
# ===========================================
#
# ``constrained_layout`` usually adjusts the axes positions on each draw
# of the figure.  If you want to get the spacing provided by
# ``constrained_layout`` but not have it update, then do the initial
# draw and then call ``fig.set_constrained_layout(False)``.
# This is potentially useful for animations where the tick labels may
# change length.
#
# Note that ``constrained_layout`` is turned off for ``ZOOM`` and ``PAN``
# GUI events for the backends that use the toolbar.  This prevents the
# axes from changing position during zooming and panning.
#
#
# Limitations
# ===========
#
# Incompatible functions
# ----------------------
#
# ``constrained_layout`` will work with `.pyplot.subplot`, but only if the
# number of rows and columns is the same for each call.
# The reason is that each call to `.pyplot.subplot` will create a new
# `.GridSpec` instance if the geometry is not the same, and
# ``constrained_layout``.  So the following works fine:

fig = plt.figure()

ax1 = plt.subplot(2, 2, 1)
ax2 = plt.subplot(2, 2, 3)
# third axes that spans both rows in second column:
ax3 = plt.subplot(2, 2, (2, 4))

example_plot(ax1)
example_plot(ax2)
example_plot(ax3)
plt.suptitle('Homogenous nrows, ncols')

###############################################################################
# but the following leads to a poor layout:

fig = plt.figure()

ax1 = plt.subplot(2, 2, 1)
ax2 = plt.subplot(2, 2, 3)
ax3 = plt.subplot(1, 2, 2)

example_plot(ax1)
example_plot(ax2)
example_plot(ax3)
plt.suptitle('Mixed nrows, ncols')

###############################################################################
# Similarly,
# `~matplotlib.pyplot.subplot2grid` works with the same limitation
# that nrows and ncols cannot change for the layout to look good.

fig = plt.figure()

ax1 = plt.subplot2grid((3, 3), (0, 0))
ax2 = plt.subplot2grid((3, 3), (0, 1), colspan=2)
ax3 = plt.subplot2grid((3, 3), (1, 0), colspan=2, rowspan=2)
ax4 = plt.subplot2grid((3, 3), (1, 2), rowspan=2)

example_plot(ax1)
example_plot(ax2)
example_plot(ax3)
example_plot(ax4)
fig.suptitle('subplot2grid')

###############################################################################
# Other Caveats
# -------------
#
# * ``constrained_layout`` only considers ticklabels, axis labels, titles, and
#   legends.  Thus, other artists may be clipped and also may overlap.
#
# * It assumes that the extra space needed for ticklabels, axis labels,
#   and titles is independent of original location of axes. This is
#   often true, but there are rare cases where it is not.
#
# * There are small differences in how the backends handle rendering fonts,
#   so the results will not be pixel-identical.
#
# * An artist using axes coordinates that extend beyond the axes
#   boundary will result in unusual layouts when added to an
#   axes. This can be avoided by adding the artist directly to the
#   :class:`~matplotlib.figure.Figure` using
#   :meth:`~matplotlib.figure.Figure.add_artist`. See
#   :class:`~matplotlib.patches.ConnectionPatch` for an example.

###########################################################
# Debugging
# =========
#
# Constrained-layout can fail in somewhat unexpected ways.  Because it uses
# a constraint solver the solver can find solutions that are mathematically
# correct, but that aren't at all what the user wants.  The usual failure
# mode is for all sizes to collapse to their smallest allowable value. If
# this happens, it is for one of two reasons:
#
# 1. There was not enough room for the elements you were requesting to draw.
# 2. There is a bug - in which case open an issue at
#    https://github.com/matplotlib/matplotlib/issues.
#
# If there is a bug, please report with a self-contained example that does
# not require outside data or dependencies (other than numpy).

###########################################################
# Notes on the algorithm
# ======================
#
# The algorithm for the constraint is relatively straightforward, but
# has some complexity due to the complex ways we can layout a figure.
#
# Layout in Matplotlib is carried out with gridspecs
# via the `~.GridSpec` class. A gridspec is a logical division of the figure
# into rows and columns, with the relative width of the Axes in those
# rows and columns set by *width_ratios* and *height_ratios*.
#
# In constrained_layout, each gridspec gets a *layoutgrid* associated with
# it.  The *layoutgrid* has a series of ``left`` and ``right`` variables
# for each column, and ``bottom`` and ``top`` variables for each row, and
# further it has a margin for each of left, right, bottom and top.  In each
# row, the bottom/top margins are widened until all the decorators
# in that row are accommodated.  Similarly for columns and the left/right
# margins.
#
#
# Simple case: one Axes
# ---------------------
#
# For a single Axes the layout is straight forward.  There is one parent
# layoutgrid for the figure consisting of one column and row, and
# a child layoutgrid for the gridspec that contains the axes, again
# consisting of one row and column. Space is made for the "decorations" on
# each side of the axes.  In the code, this is accomplished by the entries in
# ``do_constrained_layout()`` like::
#
#     gridspec._layoutgrid[0, 0].edit_margin_min('left',
#           -bbox.x0 + pos.x0 + w_pad)
#
# where ``bbox`` is the tight bounding box of the axes, and ``pos`` its
# position.  Note how the four margins encompass the axes decorations.

from matplotlib._layoutgrid import plot_children

fig, ax = plt.subplots(constrained_layout=True)
example_plot(ax, fontsize=24)
plot_children(fig, fig._layoutgrid)

#######################################################################
# Simple case: two Axes
# ---------------------
# When there are multiple axes they have their layouts bound in
# simple ways.  In this example the left axes has much larger decorations
# than the right, but they share a bottom margin, which is made large
# enough to accommodate the larger xlabel.   Same with the shared top
# margin.  The left and right margins are not shared, and hence are
# allowed to be different.

fig, ax = plt.subplots(1, 2, constrained_layout=True)
example_plot(ax[0], fontsize=32)
example_plot(ax[1], fontsize=8)
plot_children(fig, fig._layoutgrid, printit=False)

#######################################################################
# Two Axes and colorbar
# ---------------------
#
# A colorbar is simply another item that expands the margin of the parent
# layoutgrid cell:

fig, ax = plt.subplots(1, 2, constrained_layout=True)
im = ax[0].pcolormesh(arr, **pc_kwargs)
fig.colorbar(im, ax=ax[0], shrink=0.6)
im = ax[1].pcolormesh(arr, **pc_kwargs)
plot_children(fig, fig._layoutgrid)

#######################################################################
# Colorbar associated with a Gridspec
# -----------------------------------
#
# If a colorbar belongs to more than one cell of the grid, then
# it makes a larger margin for each:

fig, axs = plt.subplots(2, 2, constrained_layout=True)
for ax in axs.flat:
    im = ax.pcolormesh(arr, **pc_kwargs)
fig.colorbar(im, ax=axs, shrink=0.6)
plot_children(fig, fig._layoutgrid, printit=False)

#######################################################################
# Uneven sized Axes
# -----------------
#
# There are two ways to make axes have an uneven size in a
# Gridspec layout, either by specifying them to cross Gridspecs rows
# or columns, or by specifying width and height ratios.
#
# The first method is used here.  Note that the middle ``top`` and
# ``bottom`` margins are not affected by the left-hand column.  This
# is a conscious decision of the algorithm, and leads to the case where
# the two right-hand axes have the same height, but it is not 1/2 the height
# of the left-hand axes.  This is consietent with how ``gridspec`` works
# without constrained layout.

fig = plt.figure(constrained_layout=True)
gs = gridspec.GridSpec(2, 2, figure=fig)
ax = fig.add_subplot(gs[:, 0])
im = ax.pcolormesh(arr, **pc_kwargs)
ax = fig.add_subplot(gs[0, 1])
im = ax.pcolormesh(arr, **pc_kwargs)
ax = fig.add_subplot(gs[1, 1])
im = ax.pcolormesh(arr, **pc_kwargs)
plot_children(fig, fig._layoutgrid, printit=False)

#######################################################################
# One case that requires finessing is if margins do not have any artists
# constraining their width. In the case below, the right margin for column 0
# and the left margin for column 3 have no margin artists to set their width,
# so we take the maximum width of the margin widths that do have artists.
# This makes all the axes have the same size:

fig = plt.figure(constrained_layout=True)
gs = fig.add_gridspec(2, 4)
ax00 = fig.add_subplot(gs[0, 0:2])
ax01 = fig.add_subplot(gs[0, 2:])
ax10 = fig.add_subplot(gs[1, 1:3])
example_plot(ax10, fontsize=14)
plot_children(fig, fig._layoutgrid)