matplotlib · timhoffm · Jun 10, 2022 · Jan 22, 2022
diff --git a/lib/matplotlib/_constrained_layout.py b/lib/matplotlib/_constrained_layout.py
@@ -62,7 +62,8 @@
 
 ######################################################
 def do_constrained_layout(fig, h_pad, w_pad,
-                          hspace=None, wspace=None, rect=(0, 0, 1, 1)):
+                          hspace=None, wspace=None, rect=(0, 0, 1, 1),
+                          compress=False):
     """
     Do the constrained_layout.  Called at draw time in
      ``figure.constrained_layout()``
@@ -89,6 +90,11 @@ def do_constrained_layout(fig, h_pad, w_pad,
         Rectangle in figure coordinates to perform constrained layout in
         [left, bottom, width, height], each from 0-1.
 
+    compress : bool
+        Whether to shift Axes so that white space in between them is
+        removed. This is useful for simple grids of fixed-aspect Axes (e.g.
+        a grid of images).
+
     Returns
     -------
     layoutgrid : private debugging structure
@@ -124,13 +130,22 @@ def do_constrained_layout(fig, h_pad, w_pad,
         # update all the variables in the layout.
         layoutgrids[fig].update_variables()
 
+        warn_collapsed = ('constrained_layout not applied because '
+                          'axes sizes collapsed to zero.  Try making '
+                          'figure larger or axes decorations smaller.')
         if check_no_collapsed_axes(layoutgrids, fig):
             reposition_axes(layoutgrids, fig, renderer, h_pad=h_pad,
                             w_pad=w_pad, hspace=hspace, wspace=wspace)
+            if compress:
+                layoutgrids = compress_fixed_aspect(layoutgrids, fig)
+                layoutgrids[fig].update_variables()
+                if check_no_collapsed_axes(layoutgrids, fig):
+                    reposition_axes(layoutgrids, fig, renderer, h_pad=h_pad,
+                                    w_pad=w_pad, hspace=hspace, wspace=wspace)
+                else:
+                    _api.warn_external(warn_collapsed)
         else:
-            _api.warn_external('constrained_layout not applied because '
-                               'axes sizes collapsed to zero.  Try making '
-                               'figure larger or axes decorations smaller.')
+            _api.warn_external(warn_collapsed)
         reset_margins(layoutgrids, fig)
     return layoutgrids
 
@@ -248,6 +263,43 @@ def check_no_collapsed_axes(layoutgrids, fig):
     return True
 
 
+def compress_fixed_aspect(layoutgrids, fig):
+    gs = None
+    for ax in fig.axes:
+        if not hasattr(ax, 'get_subplotspec'):
+            continue
+        ax.apply_aspect()
+        sub = ax.get_subplotspec()
+        _gs = sub.get_gridspec()
+        if gs is None:
+            gs = _gs
+            extraw = np.zeros(gs.ncols)
+            extrah = np.zeros(gs.nrows)
+        elif _gs != gs:
+            raise ValueError('Cannot do compressed layout if axes are not'
+                                'all from the same gridspec')
+        orig = ax.get_position(original=True)
+        actual = ax.get_position(original=False)
+        dw = orig.width - actual.width
+        if dw > 0:
+            extraw[sub.colspan] = np.maximum(extraw[sub.colspan], dw)
+        dh = orig.height - actual.height
+        if dh > 0:
+            extrah[sub.rowspan] = np.maximum(extrah[sub.rowspan], dh)
+
+    if gs is None:
+        raise ValueError('Cannot do compressed layout if no axes '
+                         'are part of a gridspec.')
+    w = np.sum(extraw) / 2
+    layoutgrids[fig].edit_margin_min('left', w)
+    layoutgrids[fig].edit_margin_min('right', w)
+
+    h = np.sum(extrah) / 2
+    layoutgrids[fig].edit_margin_min('top', h)
+    layoutgrids[fig].edit_margin_min('bottom', h)
+    return layoutgrids
+
+
 def get_margin_from_padding(obj, *, w_pad=0, h_pad=0,
                             hspace=0, wspace=0):
 

diff --git a/lib/matplotlib/_layoutgrid.py b/lib/matplotlib/_layoutgrid.py
@@ -519,7 +519,7 @@ def plot_children(fig, lg=None, level=0, printit=False):
     import matplotlib.patches as mpatches
 
     if lg is None:
-        _layoutgrids = fig.execute_constrained_layout()
+        _layoutgrids = fig.get_layout_engine().execute(fig)
         lg = _layoutgrids[fig]
     colors = plt.rcParams["axes.prop_cycle"].by_key()["color"]
     col = colors[level]

diff --git a/lib/matplotlib/figure.py b/lib/matplotlib/figure.py
@@ -2245,7 +2245,7 @@ def __init__(self,
                 The use of this parameter is discouraged. Please use
                 ``layout='constrained'`` instead.
 
-        layout : {'constrained', 'tight', `.LayoutEngine`, None}, optional
+        layout : {'constrained', 'compressed', 'tight', `.LayoutEngine`, None}
             The layout mechanism for positioning of plot elements to avoid
             overlapping Axes decorations (labels, ticks, etc). Note that
             layout managers can have significant performance penalties.
@@ -2258,6 +2258,10 @@ def __init__(self,
               See :doc:`/tutorials/intermediate/constrainedlayout_guide`
               for examples.
 
+            - 'compressed': uses the same algorithm as 'constrained', but
+              removes extra space between fixed-aspect-ratio Axes.  Best for
+              simple grids of axes.
+
             - 'tight': Use the tight layout mechanism. This is a relatively
               simple algorithm that adjusts the subplot parameters so that
               decorations do not overlap. See `.Figure.set_tight_layout` for
@@ -2388,11 +2392,13 @@ def set_layout_engine(self, layout=None, **kwargs):
 
         Parameters
         ----------
-        layout : {'constrained', 'tight'} or `~.LayoutEngine`
-            'constrained' will use `~.ConstrainedLayoutEngine`, 'tight' will
-            use `~.TightLayoutEngine`.  Users and libraries can define their
-            own layout engines as well.
-        kwargs : dict
+        layout: {'constrained', 'compressed', 'tight'} or `~.LayoutEngine`
+            'constrained' will use `~.ConstrainedLayoutEngine`,
+            'compressed' will also use ConstrainedLayoutEngine, but with a
+            correction that attempts to make a good layout for fixed-aspect
+            ratio Axes. 'tight' uses `~.TightLayoutEngine`.  Users and
+            libraries can define their own layout engines as well.
+        kwargs: dict
             The keyword arguments are passed to the layout engine to set things
             like padding and margin sizes.  Only used if *layout* is a string.
         """
@@ -2408,6 +2414,9 @@ def set_layout_engine(self, layout=None, **kwargs):
             new_layout_engine = TightLayoutEngine(**kwargs)
         elif layout == 'constrained':
             new_layout_engine = ConstrainedLayoutEngine(**kwargs)
+        elif layout == 'compressed':
+            new_layout_engine = ConstrainedLayoutEngine(compress=True,
+                                                        **kwargs)
         elif isinstance(layout, LayoutEngine):
             new_layout_engine = layout
         else:

diff --git a/lib/matplotlib/layout_engine.py b/lib/matplotlib/layout_engine.py
@@ -180,7 +180,7 @@ class ConstrainedLayoutEngine(LayoutEngine):
 
     def __init__(self, *, h_pad=None, w_pad=None,
                  hspace=None, wspace=None, rect=(0, 0, 1, 1),
-                 **kwargs):
+                 compress=False, **kwargs):
         """
         Initialize ``constrained_layout`` settings.
 
@@ -201,6 +201,10 @@ def __init__(self, *, h_pad=None, w_pad=None,
         rect : tuple of 4 floats
             Rectangle in figure coordinates to perform constrained layout in
             (left, bottom, width, height), each from 0-1.
+        compress : bool
+            Whether to shift Axes so that white space in between them is
+            removed. This is useful for simple grids of fixed-aspect Axes (e.g.
+            a grid of images).  See :ref:`compressed_layout`.
         """
         super().__init__(**kwargs)
         # set the defaults:
@@ -212,6 +216,7 @@ def __init__(self, *, h_pad=None, w_pad=None,
         # set anything that was passed in (None will be ignored):
         self.set(w_pad=w_pad, h_pad=h_pad, wspace=wspace, hspace=hspace,
                  rect=rect)
+        self._compress = compress
 
     def execute(self, fig):
         """
@@ -229,7 +234,8 @@ def execute(self, fig):
         return do_constrained_layout(fig, w_pad=w_pad, h_pad=h_pad,
                                      wspace=self._params['wspace'],
                                      hspace=self._params['hspace'],
-                                     rect=self._params['rect'])
+                                     rect=self._params['rect'],
+                                     compress=self._compress)
 
     def set(self, *, h_pad=None, w_pad=None,
             hspace=None, wspace=None, rect=None):

diff --git a/lib/matplotlib/tests/test_constrainedlayout.py b/lib/matplotlib/tests/test_constrainedlayout.py
@@ -624,3 +624,34 @@ def test_rect():
     assert ppos.y1 < 0.5
     assert ppos.x0 > 0.2
     assert ppos.y0 > 0.2
+
+
+def test_compressed1():
+    fig, axs = plt.subplots(3, 2, layout='compressed',
+                            sharex=True, sharey=True)
+    for ax in axs.flat:
+        pc = ax.imshow(np.random.randn(20, 20))
+
+    fig.colorbar(pc, ax=axs)
+    fig.draw_without_rendering()
+
+    pos = axs[0, 0].get_position()
+    np.testing.assert_allclose(pos.x0, 0.2344, atol=1e-3)
+    pos = axs[0, 1].get_position()
+    np.testing.assert_allclose(pos.x1, 0.7024, atol=1e-3)
+
+    # wider than tall
+    fig, axs = plt.subplots(2, 3, layout='compressed',
+                            sharex=True, sharey=True, figsize=(5, 4))
+    for ax in axs.flat:
+        pc = ax.imshow(np.random.randn(20, 20))
+
+    fig.colorbar(pc, ax=axs)
+    fig.draw_without_rendering()
+
+    pos = axs[0, 0].get_position()
+    np.testing.assert_allclose(pos.x0, 0.06195, atol=1e-3)
+    np.testing.assert_allclose(pos.y1, 0.8537, atol=1e-3)
+    pos = axs[1, 2].get_position()
+    np.testing.assert_allclose(pos.x1, 0.8618, atol=1e-3)
+    np.testing.assert_allclose(pos.y0, 0.1934, atol=1e-3)
diff --git a/tutorials/intermediate/arranging_axes.py b/tutorials/intermediate/arranging_axes.py
@@ -100,7 +100,7 @@
 import numpy as np
 
 fig, axs = plt.subplots(ncols=2, nrows=2, figsize=(5.5, 3.5),
-                        constrained_layout=True)
+                        layout="constrained")
 # add an artist, in this case a nice label in the middle...
 for row in range(2):
     for col in range(2):
@@ -129,11 +129,41 @@ def annotate_axes(ax, text, fontsize=18):
 
 fig, axd = plt.subplot_mosaic([['upper left', 'upper right'],
                                ['lower left', 'lower right']],
-                              figsize=(5.5, 3.5), constrained_layout=True)
+                              figsize=(5.5, 3.5), layout="constrained")
 for k in axd:
     annotate_axes(axd[k], f'axd["{k}"]', fontsize=14)
 fig.suptitle('plt.subplot_mosaic()')
 
+#############################################################################
+#
+# Grids of fixed-aspect ratio Axes
+# --------------------------------
+#
+# Fixed-aspect ratio axes are common for images or maps.  However, they
+# present a challenge to layout because two sets of constraints are being
+# imposed on the size of the Axes - that they fit in the figure and that they
+# have a set aspect ratio.  This leads to large gaps between Axes by default:
+#
+
+fig, axs = plt.subplots(2, 2, layout="constrained", figsize=(5.5, 3.5))
+for ax in axs.flat:
+    ax.set_aspect(1)
+fig.suptitle('Fixed aspect Axes')
+
+############################################################################
+# One way to address this is to change the aspect of the figure to be close
+# to the aspect ratio of the Axes, however that requires trial and error.
+# Matplotlib also supplies ``layout="compressed"``, which will work with
+# simple grids to reduce the gaps between Axes.  (The ``mpl_toolkits`` also
+# provides `~.mpl_toolkits.axes_grid1.axes_grid.ImageGrid` to accomplish
+# a similar effect, but with a non-standard Axes class).
+
+fig, axs = plt.subplots(2, 2, layout="compressed", figsize=(5.5, 3.5))
+for ax in axs.flat:
+    ax.set_aspect(1)
+fig.suptitle('Fixed aspect Axes: compressed')
+
+
 ############################################################################
 # Axes spanning rows or columns in a grid
 # ---------------------------------------
@@ -145,7 +175,7 @@ def annotate_axes(ax, text, fontsize=18):
 
 fig, axd = plt.subplot_mosaic([['upper left', 'right'],
                                ['lower left', 'right']],
-                              figsize=(5.5, 3.5), constrained_layout=True)
+                              figsize=(5.5, 3.5), layout="constrained")
 for k in axd:
     annotate_axes(axd[k], f'axd["{k}"]', fontsize=14)
 fig.suptitle('plt.subplot_mosaic()')
@@ -168,7 +198,7 @@ def annotate_axes(ax, text, fontsize=18):
 fig, axd = plt.subplot_mosaic([['upper left', 'right'],
                                ['lower left', 'right']],
                               gridspec_kw=gs_kw, figsize=(5.5, 3.5),
-                              constrained_layout=True)
+                              layout="constrained")
 for k in axd:
     annotate_axes(axd[k], f'axd["{k}"]', fontsize=14)
 fig.suptitle('plt.subplot_mosaic()')
@@ -184,7 +214,7 @@ def annotate_axes(ax, text, fontsize=18):
 # necessarily aligned.  See below for a more verbose way to achieve the same
 # effect with `~.gridspec.GridSpecFromSubplotSpec`.
 
-fig = plt.figure(constrained_layout=True)
+fig = plt.figure(layout="constrained")
 subfigs = fig.subfigures(1, 2, wspace=0.07, width_ratios=[1.5, 1.])
 axs0 = subfigs[0].subplots(2, 2)
 subfigs[0].set_facecolor('0.9')
@@ -207,7 +237,7 @@ def annotate_axes(ax, text, fontsize=18):
 outer = [['upper left',  inner],
           ['lower left', 'lower right']]
 
-fig, axd = plt.subplot_mosaic(outer, constrained_layout=True)
+fig, axd = plt.subplot_mosaic(outer, layout="constrained")
 for k in axd:
     annotate_axes(axd[k], f'axd["{k}"]')
 
@@ -230,7 +260,7 @@ def annotate_axes(ax, text, fontsize=18):
 # We can accomplish a 2x2 grid in the same manner as
 # ``plt.subplots(2, 2)``:
 
-fig = plt.figure(figsize=(5.5, 3.5), constrained_layout=True)
+fig = plt.figure(figsize=(5.5, 3.5), layout="constrained")
 spec = fig.add_gridspec(ncols=2, nrows=2)
 
 ax0 = fig.add_subplot(spec[0, 0])
@@ -256,7 +286,7 @@ def annotate_axes(ax, text, fontsize=18):
 # and the new Axes will span the slice.  This would be the same
 # as ``fig, axd = plt.subplot_mosaic([['ax0', 'ax0'], ['ax1', 'ax2']], ...)``:
 
-fig = plt.figure(figsize=(5.5, 3.5), constrained_layout=True)
+fig = plt.figure(figsize=(5.5, 3.5), layout="constrained")
 spec = fig.add_gridspec(2, 2)
 
 ax0 = fig.add_subplot(spec[0, :])
@@ -284,7 +314,7 @@ def annotate_axes(ax, text, fontsize=18):
 # These spacing parameters can also be passed to `~.pyplot.subplots` and
 # `~.pyplot.subplot_mosaic` as the *gridspec_kw* argument.
 
-fig = plt.figure(constrained_layout=False, facecolor='0.9')
+fig = plt.figure(layout=None, facecolor='0.9')
 gs = fig.add_gridspec(nrows=3, ncols=3, left=0.05, right=0.75,
                       hspace=0.1, wspace=0.05)
 ax0 = fig.add_subplot(gs[:-1, :])
@@ -306,7 +336,7 @@ def annotate_axes(ax, text, fontsize=18):
 # Note this is also available from the more verbose
 # `.gridspec.GridSpecFromSubplotSpec`.
 
-fig = plt.figure(constrained_layout=True)
+fig = plt.figure(layout="constrained")
 gs0 = fig.add_gridspec(1, 2)
 
 gs00 = gs0[0].subgridspec(2, 2)