Properly position markers in step plots.

anntzer · anntzer · commit 3df886b0f58b · 2018-06-10T10:01:19.000+02:00
While it may appear costly to recache the path at every draw, this only
occurs when using step plots with markers (a relatively rare case) and
is still better than before the bug was introduced, when step plots used
to need to be recomputed at every draw, regardless of whether markers
are present.
diff --git a/lib/matplotlib/lines.py b/lib/matplotlib/lines.py
@@ -739,16 +739,17 @@ def draw(self, renderer):
             subslice = slice(max(i0 - 1, 0), i1 + 1)
             self.ind_offset = subslice.start
             self._transform_path(subslice)
-
-        transf_path = self._get_transformed_path()
+        else:
+            subslice = None
 
         if self.get_path_effects():
             from matplotlib.patheffects import PathEffectRenderer
             renderer = PathEffectRenderer(self.get_path_effects(), renderer)
 
         renderer.open_group('line2d', self.get_gid())
         if self._lineStyles[self._linestyle] != '_draw_nothing':
-            tpath, affine = transf_path.get_transformed_path_and_affine()
+            tpath, affine = (self._get_transformed_path()
+                             .get_transformed_path_and_affine())
             if len(tpath.vertices):
                 gc = renderer.new_gc()
                 self._set_gc_clip(gc)
@@ -796,7 +797,20 @@ def draw(self, renderer):
             gc.set_foreground(ec_rgba, isRGBA=True)
 
             marker = self._marker
-            tpath, affine = transf_path.get_transformed_points_and_affine()
+
+            # Markers *must* be drawn ignoring the drawstyle (but don't pay the
+            # recaching if drawstyle is already "default").
+            if self.get_drawstyle() != "default":
+                with cbook._setattr_cm(
+                        self, _drawstyle="default", _transformed_path=None):
+                    self.recache()
+                    self._transform_path(subslice)
+                    tpath, affine = (self._get_transformed_path()
+                                    .get_transformed_path_and_affine())
+            else:
+                tpath, affine = (self._get_transformed_path()
+                                 .get_transformed_path_and_affine())
+
             if len(tpath.vertices):
                 # subsample the markers if markevery is not None
                 markevery = self.get_markevery()
diff --git a/lib/matplotlib/tests/test_lines.py b/lib/matplotlib/tests/test_lines.py
@@ -1,6 +1,8 @@
 """
 Tests specific to the lines module.
 """
+
+from io import BytesIO
 import itertools
 import matplotlib.lines as mlines
 import pytest
@@ -195,3 +197,15 @@ def test_nan_is_sorted():
     assert line._is_sorted(np.array([1, 2, 3]))
     assert line._is_sorted(np.array([1, np.nan, 3]))
     assert not line._is_sorted([3, 5] + [np.nan] * 100 + [0, 2])
+
+
+def test_step_markers():
+    fig, ax = plt.subplots()
+    ax.step([0, 1], "-o")
+    buf1 = BytesIO()
+    fig.savefig(buf1)
+    fig, ax = plt.subplots()
+    ax.plot([0, 0, 1], [0, 1, 1], "-o", markevery=[0, 2])
+    buf2 = BytesIO()
+    fig.savefig(buf2)
+    assert buf1.getvalue() == buf2.getvalue()
