Make set_x/ymargin() update axes limits, just like margins().

anntzer · anntzer · commit b5fd761a03ff · 2020-03-10T21:43:21.000+01:00
Previously, setting the margins via set_x/ymargin() would not update
axes limits, unlike calling margins().  This appears to be a simple
oversight.

The change in collections.py is necessary because the difference in
timing at which autoscale is executed now exposed a floating point
inaccuracy (which gets amplified by the old round_numbers autolimits
mode).
diff --git a/lib/matplotlib/axes/_base.py b/lib/matplotlib/axes/_base.py
@@ -2239,6 +2239,7 @@ def set_xmargin(self, m):
         if m <= -0.5:
             raise ValueError("margin must be greater than -0.5")
         self._xmargin = m
+        self._request_autoscale_view(scalex=True, scaley=False)
         self.stale = True
 
     def set_ymargin(self, m):
@@ -2261,6 +2262,7 @@ def set_ymargin(self, m):
         if m <= -0.5:
             raise ValueError("margin must be greater than -0.5")
         self._ymargin = m
+        self._request_autoscale_view(scalex=False, scaley=True)
         self.stale = True
 
     def margins(self, *margins, x=None, y=None, tight=True):
@@ -2331,15 +2333,13 @@ def margins(self, *margins, x=None, y=None, tight=True):
                 cbook._warn_external(f'ignoring tight={tight!r} in get mode')
             return self._xmargin, self._ymargin
 
+        if tight is not None:
+            self._tight = tight
         if x is not None:
             self.set_xmargin(x)
         if y is not None:
             self.set_ymargin(y)
 
-        self._request_autoscale_view(
-            tight=tight, scalex=(x is not None), scaley=(y is not None)
-        )
-
     def set_rasterization_zorder(self, z):
         """
         Parameters
diff --git a/lib/matplotlib/collections.py b/lib/matplotlib/collections.py
@@ -213,8 +213,6 @@ def get_datalim(self, transData):
             # we may have transform.contains_branch(transData) but not
             # transforms.get_affine().contains_branch(transData).  But later,
             # be careful to only apply the affine part that remains.
-        if not transOffset.is_affine:
-            offsets = transOffset.transform_non_affine(offsets)
 
         if isinstance(offsets, np.ma.MaskedArray):
             offsets = offsets.filled(np.nan)
@@ -228,17 +226,18 @@ def get_datalim(self, transData):
                 # also use this algorithm (like streamplot).
                 result = mpath.get_path_collection_extents(
                     transform.get_affine(), paths, self.get_transforms(),
-                    offsets, transOffset.get_affine().frozen())
+                    transOffset.transform_non_affine(offsets),
+                    transOffset.get_affine().frozen())
                 return result.inverse_transformed(transData)
             if not self._offsetsNone:
-                # this is for collections that have their paths (shapes)
-                # in physical, axes-relative, or figure-relative units
-                # (i.e. like scatter). We can't uniquely set limits based on
-                # those shapes, so we just set the limits based on their
-                # location.
-                # Finish the transform:
-                offsets = (transOffset.get_affine() +
-                           transData.inverted()).transform(offsets)
+                # This is for collections that have their paths (shapes) in
+                # physical, axes-relative, or figure-relative units (i.e.
+                # like scatter).  We can't uniquely set limits based on those
+                # shapes, so we just set the limits based on their location.
+                # It is important to use "transOffset - transData" here so that
+                # if transOffset == transData, then they exactly cancel out,
+                # rather than possibly accumulating floating point error.
+                offsets = (transOffset - transData).transform(offsets)
                 offsets = np.ma.masked_invalid(offsets)
                 if not offsets.mask.all():
                     points = np.row_stack((offsets.min(axis=0),
diff --git a/lib/matplotlib/tests/test_axes.py b/lib/matplotlib/tests/test_axes.py
@@ -5346,6 +5346,14 @@ def test_margins():
                               ymax + (ymax - ymin) * 0.5)
 
 
+def test_set_margin_updates_limits():
+    mpl.style.use("default")
+    fig, ax = plt.subplots()
+    ax.plot([1, 2], [1, 2])
+    ax.set(xscale="log", xmargin=0)
+    assert ax.get_xlim() == (1, 2)
+
+
 def test_length_one_hist():
     fig, ax = plt.subplots()
     ax.hist(1)
