matplotlib · oscargus · May 8, 2022 · Mar 9, 2022 · oscargus · Mar 13, 2022
diff --git a/lib/matplotlib/_constrained_layout.py b/lib/matplotlib/_constrained_layout.py
@@ -64,7 +64,7 @@
 
 ######################################################
 def do_constrained_layout(fig, h_pad, w_pad,
-                          hspace=None, wspace=None):
+                          hspace=None, wspace=None, rect=(0, 0, 1, 1)):
     """
     Do the constrained_layout.  Called at draw time in
      ``figure.constrained_layout()``
@@ -87,14 +87,18 @@ def do_constrained_layout(fig, h_pad, w_pad,
        of 0.1 of the figure width between each column.
        If h/wspace < h/w_pad, then the pads are used instead.
 
+    rect : tuple of 4 floats
+        Rectangle in figure coordinates to perform constrained layout in
+        [left, bottom, width, height], each from 0-1.
-        [left, bottom, width, height], each from 0-1.
+        (left, bottom, width, height), each from 0-1.
-        [left, bottom, width, height], each from 0-1.
+        (left, bottom, width, height), each from 0-1.
+
     Returns
     -------
     layoutgrid : private debugging structure
     """
 
     renderer = get_renderer(fig)
     # make layoutgrid tree...
-    layoutgrids = make_layoutgrids(fig, None)
+    layoutgrids = make_layoutgrids(fig, None, rect=rect)
     if not layoutgrids['hasgrids']:
         _api.warn_external('There are no gridspecs with layoutgrids. '
                            'Possibly did not call parent GridSpec with the'
@@ -133,7 +137,7 @@ def do_constrained_layout(fig, h_pad, w_pad,
     return layoutgrids
 
 
-def make_layoutgrids(fig, layoutgrids):
+def make_layoutgrids(fig, layoutgrids, rect=(0, 0, 1, 1)):
     """
     Make the layoutgrid tree.
 
@@ -147,8 +151,9 @@ def make_layoutgrids(fig, layoutgrids):
         layoutgrids = dict()
         layoutgrids['hasgrids'] = False
     if not hasattr(fig, '_parent'):
-        # top figure
-        layoutgrids[fig] = mlayoutgrid.LayoutGrid(parent=None, name='figlb')
+        # top figure;  pass rect as parent to allow user-specified
+        # margins
+        layoutgrids[fig] = mlayoutgrid.LayoutGrid(parent=rect, name='figlb')
     else:
         # subfigure
         gs = fig._subplotspec.get_gridspec()

diff --git a/lib/matplotlib/_layoutgrid.py b/lib/matplotlib/_layoutgrid.py
@@ -39,7 +39,7 @@ def __init__(self, parent=None, parent_pos=(0, 0),
         self.parent_pos = parent_pos
         self.parent_inner = parent_inner
         self.name = name + seq_id()
-        if parent is not None:
+        if isinstance(parent, LayoutGrid):
             self.name = f'{parent.name}.{self.name}'
         self.nrows = nrows
         self.ncols = ncols
@@ -51,8 +51,10 @@ def __init__(self, parent=None, parent_pos=(0, 0),
             self.width_ratios = np.ones(ncols)
 
         sn = self.name + '_'
-        if parent is None:
-            self.parent = None
+        if not isinstance(parent, LayoutGrid):
+            # parent can be a rect if not a LayoutGrid
+            # allows specifying a rectangle to contain the layout.
+            self.parent = parent
             self.solver = kiwi.Solver()
         else:
             self.parent = parent
@@ -178,12 +180,13 @@ def parent_constraints(self):
         # parent's left, the last column right equal to the
         # parent's right...
         parent = self.parent
-        if parent is None:
-            hc = [self.lefts[0] == 0,
-                  self.rights[-1] == 1,
+        if not isinstance(parent, LayoutGrid):
+            # specify a rectangle in figure coordinates
+            hc = [self.lefts[0] == parent[0],
+                  self.rights[-1] == parent[0] + parent[2],
                   # top and bottom reversed order...
-                  self.tops[0] == 1,
-                  self.bottoms[-1] == 0]
+                  self.tops[0] == parent[1] + parent[3],
+                  self.bottoms[-1] == parent[1]]
         else:
             rows, cols = self.parent_pos
             rows = np.atleast_1d(rows)

diff --git a/lib/matplotlib/layout_engine.py b/lib/matplotlib/layout_engine.py
@@ -123,7 +123,7 @@ def __init__(self, *, pad=1.08, h_pad=None, w_pad=None,
         h_pad, w_pad : float
             Padding (height/width) between edges of adjacent subplots.
             Defaults to *pad*.
-        rect : tuple[float, float, float, float], optional
+        rect : tuple of 4 floats, optional
             (left, bottom, right, top) rectangle in normalized figure
             coordinates that the subplots (including labels)
             will fit into. Defaults to using the entire figure.
@@ -179,7 +179,8 @@ class ConstrainedLayoutEngine(LayoutEngine):
     _colorbar_gridspec = False
 
     def __init__(self, *, h_pad=None, w_pad=None,
-                 hspace=None, wspace=None, **kwargs):
+                 hspace=None, wspace=None, rect=(0, 0, 1, 1),
+                 **kwargs):
         """
         Initialize ``constrained_layout`` settings.
 
@@ -197,15 +198,20 @@ def __init__(self, *, h_pad=None, w_pad=None,
             If h/wspace < h/w_pad, then the pads are used instead.
             Default to :rc:`figure.constrained_layout.hspace` and
             :rc:`figure.constrained_layout.wspace`.
+        rect : tuple of 4 floats
+            Rectangle in figure coordinates to perform constrained layout in
+            (left, bottom, width, height), each from 0-1.
         """
         super().__init__(**kwargs)
         # set the defaults:
         self.set(w_pad=mpl.rcParams['figure.constrained_layout.w_pad'],
                  h_pad=mpl.rcParams['figure.constrained_layout.h_pad'],
                  wspace=mpl.rcParams['figure.constrained_layout.wspace'],
-                 hspace=mpl.rcParams['figure.constrained_layout.hspace'])
+                 hspace=mpl.rcParams['figure.constrained_layout.hspace'],
+                 rect=(0, 0, 1, 1))
         # set anything that was passed in (None will be ignored):
-        self.set(w_pad=w_pad, h_pad=h_pad, wspace=wspace, hspace=hspace)
+        self.set(w_pad=w_pad, h_pad=h_pad, wspace=wspace, hspace=hspace,
+                 rect=rect)
 
     def execute(self, fig):
         """
@@ -222,10 +228,11 @@ 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'])
+                                     hspace=self._params['hspace'],
+                                     rect=self._params['rect'])
 
     def set(self, *, h_pad=None, w_pad=None,
-            hspace=None, wspace=None):
+            hspace=None, wspace=None, rect=None):
         """
         Set the pads for constrained_layout.
 
@@ -243,6 +250,9 @@ def set(self, *, h_pad=None, w_pad=None,
             If h/wspace < h/w_pad, then the pads are used instead.
             Default to :rc:`figure.constrained_layout.hspace` and
             :rc:`figure.constrained_layout.wspace`.
+        rect : tuple of 4 floats
+            Rectangle in figure coordinates to perform constrained layout in
+            (left, bottom, width, height), each from 0-1.
         """
         for td in self.set.__kwdefaults__:
             if locals()[td] is not None:

diff --git a/lib/matplotlib/tests/test_constrainedlayout.py b/lib/matplotlib/tests/test_constrainedlayout.py
@@ -608,3 +608,21 @@ def test_discouraged_api():
 def test_kwargs():
     fig, ax = plt.subplots(constrained_layout={'h_pad': 0.02})
     fig.draw_without_rendering()
+
+
+def test_rect():
+    fig, ax = plt.subplots(layout='constrained')
+    fig.get_layout_engine().set(rect=[0, 0, 0.5, 0.5])
+    fig.draw_without_rendering()
+    ppos = ax.get_position()
+    assert ppos.x1 < 0.5
+    assert ppos.y1 < 0.5
+
+    fig, ax = plt.subplots(layout='constrained')
+    fig.get_layout_engine().set(rect=[0.2, 0.2, 0.3, 0.3])
+    fig.draw_without_rendering()
+    ppos = ax.get_position()
+    assert ppos.x1 < 0.5
+    assert ppos.y1 < 0.5
+    assert ppos.x0 > 0.2
+    assert ppos.y0 > 0.2
diff --git a/pytest.ini b/pytest.ini
diff --git a/tests.py b/tests.py