Reuse InsetLocator to make twinned axes follow their parents.

anntzer · anntzer · commit 3490c90bc48c · 2020-09-11T14:24:57.000+02:00
From a positioning PoV, a twinned axes is just like an inset axes whose position exactly matches the parent's position. Doing so removes the need for the heuristic in `Figure.subplots_adjust`/`GridSpec.update` where a non-gridspec-managed Axes would track the position of any Axes with which it shared either xaxis or yaxis, which was a proxy for twinning (per 721b949). This would cause incorrect behavior in rare cases such as ```python from pylab import * ax = subplot(221) axes([.6, .6, .3, .3], sharex=ax) subplots_adjust(left=0) ``` where the `subplots_adjust` call would make the second axes go on top of the first.
diff --git a/lib/matplotlib/axes/_axes.py b/lib/matplotlib/axes/_axes.py
@@ -31,38 +31,14 @@
 import matplotlib.tri as mtri
 import matplotlib.units as munits
 from matplotlib import _preprocess_data, rcParams
-from matplotlib.axes._base import _AxesBase, _process_plot_format
+from matplotlib.axes._base import (
+    _AxesBase, _InsetLocator, _process_plot_format)
 from matplotlib.axes._secondary_axes import SecondaryAxis
 from matplotlib.container import BarContainer, ErrorbarContainer, StemContainer
 
 _log = logging.getLogger(__name__)
 
 
-class _InsetLocator:
-    """
-    Axes locator for `.Axes.inset_axes`.
-
-    The locater is a callable object used in `.Axes.set_aspect` to compute the
-    axes location depending on the renderer.
-    """
-
-    def __init__(self, bounds, transform):
-        """
-        *bounds* (a ``[l, b, w, h]`` rectangle) and *transform* together
-        specify the position of the inset axes.
-        """
-        self._bounds = bounds
-        self._transform = transform
-
-    def __call__(self, ax, renderer):
-        # Subtracting transFigure will typically rely on inverted(), freezing
-        # the transform; thus, this needs to be delayed until draw time as
-        # transFigure may otherwise change after this is evaluated.
-        return mtransforms.TransformedBbox(
-            mtransforms.Bbox.from_bounds(*self._bounds),
-            self._transform - ax.figure.transFigure)
-
-
 # The axes module contains all the wrappers to plotting functions.
 # All the other methods should go in the _AxesBase class.
 
diff --git a/lib/matplotlib/axes/_base.py b/lib/matplotlib/axes/_base.py
@@ -85,6 +85,31 @@ def wrapper(self, *args, **kwargs):
         setattr(owner, name, wrapper)
 
 
+class _InsetLocator:
+    """
+    Axes locator for `.Axes.inset_axes` and similarly positioned axes.
+
+    The locater is a callable object used in `.Axes.set_aspect` to compute the
+    axes location depending on the renderer.
+    """
+
+    def __init__(self, bounds, transform):
+        """
+        *bounds* (a ``[l, b, w, h]`` rectangle) and *transform* together
+        specify the position of the inset axes.
+        """
+        self._bounds = bounds
+        self._transform = transform
+
+    def __call__(self, ax, renderer):
+        # Subtracting transFigure will typically rely on inverted(), freezing
+        # the transform; thus, this needs to be delayed until draw time as
+        # transFigure may otherwise change after this is evaluated.
+        return mtransforms.TransformedBbox(
+            mtransforms.Bbox.from_bounds(*self._bounds),
+            self._transform - ax.figure.transFigure)
+
+
 def _process_plot_format(fmt):
     """
     Convert a MATLAB style color/line style format string to a (*linestyle*,
@@ -4336,7 +4361,9 @@ def _make_twin_axes(self, *args, **kwargs):
         # Typically, SubplotBase._make_twin_axes is called instead of this.
         if 'sharex' in kwargs and 'sharey' in kwargs:
             raise ValueError("Twinned Axes may share only one axis")
-        ax2 = self.figure.add_axes(self.get_position(True), *args, **kwargs)
+        ax2 = self.figure.add_axes(
+            self.get_position(True), *args, **kwargs,
+            axes_locator=_InsetLocator([0, 0, 1, 1], self.transAxes))
         self.set_adjustable('datalim')
         ax2.set_adjustable('datalim')
         self._twinned_axes.join(self, ax2)
diff --git a/lib/matplotlib/axes/_secondary_axes.py b/lib/matplotlib/axes/_secondary_axes.py
@@ -4,7 +4,7 @@
 import matplotlib.docstring as docstring
 import matplotlib.ticker as mticker
 from matplotlib.axes import _axes
-from matplotlib.axes._base import _AxesBase
+from matplotlib.axes._base import _AxesBase, _InsetLocator
 
 
 class SecondaryAxis(_AxesBase):
@@ -114,7 +114,7 @@ def set_location(self, location):
         else:
             bounds = [self._pos, 0, 1e-10, 1]
 
-        secondary_locator = _axes._InsetLocator(bounds, self._parent.transAxes)
+        secondary_locator = _InsetLocator(bounds, self._parent.transAxes)
 
         # this locator lets the axes move in the parent axes coordinates.
         # so it never needs to know where the parent is explicitly in
diff --git a/lib/matplotlib/figure.py b/lib/matplotlib/figure.py
@@ -2424,15 +2424,7 @@ def subplots_adjust(self, left=None, bottom=None, right=None, top=None,
                                  "constrained_layout==False. ")
         self.subplotpars.update(left, bottom, right, top, wspace, hspace)
         for ax in self.axes:
-            if not isinstance(ax, SubplotBase):
-                # Check if sharing a subplots axis
-                if isinstance(ax._sharex, SubplotBase):
-                    ax._sharex.update_params()
-                    ax.set_position(ax._sharex.figbox)
-                elif isinstance(ax._sharey, SubplotBase):
-                    ax._sharey.update_params()
-                    ax.set_position(ax._sharey.figbox)
-            else:
+            if isinstance(ax, SubplotBase):
                 ax.update_params()
                 ax.set_position(ax.figbox)
         self.stale = True
diff --git a/lib/matplotlib/gridspec.py b/lib/matplotlib/gridspec.py
@@ -486,18 +486,7 @@ def update(self, **kwargs):
                 raise AttributeError(f"{k} is an unknown keyword")
         for figmanager in _pylab_helpers.Gcf.figs.values():
             for ax in figmanager.canvas.figure.axes:
-                # copied from Figure.subplots_adjust
-                if not isinstance(ax, mpl.axes.SubplotBase):
-                    # Check if sharing a subplots axis
-                    if isinstance(ax._sharex, mpl.axes.SubplotBase):
-                        if ax._sharex.get_subplotspec().get_gridspec() == self:
-                            ax._sharex.update_params()
-                            ax._set_position(ax._sharex.figbox)
-                    elif isinstance(ax._sharey, mpl.axes.SubplotBase):
-                        if ax._sharey.get_subplotspec().get_gridspec() == self:
-                            ax._sharey.update_params()
-                            ax._set_position(ax._sharey.figbox)
-                else:
+                if isinstance(ax, mpl.axes.SubplotBase):
                     ss = ax.get_subplotspec().get_topmost_subplotspec()
                     if ss.get_gridspec() == self:
                         ax.update_params()
