matplotlib · jklymak · Feb 11, 2023 · Dec 29, 2022 · rcomer · Dec 31, 2022
diff --git a/lib/matplotlib/collections.py b/lib/matplotlib/collections.py
@@ -9,6 +9,7 @@
 line segments).
 """
 
+import itertools
 import math
 from numbers import Number
 import warnings
@@ -163,6 +164,9 @@ def __init__(self,
         # list of unbroadcast/scaled linewidths
         self._us_lw = [0]
         self._linewidths = [0]
+
+        self._gapcolor = None  # Currently only used by LineCollection.
+
         # Flags set by _set_mappable_flags: are colors from mapping an array?
         self._face_is_mapped = None
         self._edge_is_mapped = None
@@ -406,6 +410,17 @@ def draw(self, renderer):
                 gc, paths[0], combined_transform.frozen(),
                 mpath.Path(offsets), offset_trf, tuple(facecolors[0]))
         else:
+            if self._gapcolor is not None:
 all(ls[1] is None for ls in self._linestyles) and 
 all(ls[1] is None for ls in self._linestyles) and 
+                # First draw paths within the gaps.
+                ipaths, ilinestyles = self._get_inverse_paths_linestyles()
+                renderer.draw_path_collection(
+                    gc, transform.frozen(), ipaths,
+                    self.get_transforms(), offsets, offset_trf,
+                    [mcolors.to_rgba("none")], self._gapcolor,
+                    self._linewidths, ilinestyles,
+                    self._antialiaseds, self._urls,
+                    "screen")
+
             renderer.draw_path_collection(
                 gc, transform.frozen(), paths,
                 self.get_transforms(), offsets, offset_trf,
@@ -1459,6 +1474,12 @@ def _get_default_edgecolor(self):
     def _get_default_facecolor(self):
         return 'none'
 
+    def set_alpha(self, alpha):
+        # docstring inherited
+        super().set_alpha(alpha)
+        if self._gapcolor is not None:
+            self.set_gapcolor(self._original_gapcolor)
+
     def set_color(self, c):
         """
         Set the edgecolor(s) of the LineCollection.
@@ -1479,6 +1500,53 @@ def get_color(self):
 
     get_colors = get_color  # for compatibility with old versions
 
+    def set_gapcolor(self, gapcolor):
+        """
+        Set a color to fill the gaps in the dashed line style.
+
+        .. note::
+
+            Striped lines are created by drawing two interleaved dashed lines.
+            There can be overlaps between those two, which may result in
+            artifacts when using transparency.
+
+            This functionality is experimental and may change.
+
+        Parameters
+        ----------
+        gapcolor : color or list of colors or None
+            The color with which to fill the gaps. If None, the gaps are
+            unfilled.
+        """
+        self._original_gapcolor = gapcolor
+        self._set_gapcolor(gapcolor)
+
+    def _set_gapcolor(self, gapcolor):
+        if gapcolor is not None:
+            gapcolor = mcolors.to_rgba_array(gapcolor, self._alpha)
+        self._gapcolor = gapcolor
+        self.stale = True
+
+    def get_gapcolor(self):
+        return self._gapcolor
+
+    def _get_inverse_paths_linestyles(self):
+        """
+        Returns the path and pattern for the gaps in the non-solid lines.
+
+        This path and pattern is the inverse of the path and pattern used to
+        construct the non-solid lines. For solid lines, we set the inverse path
+        to nans to prevent drawing an inverse line.
+        """
+        path_patterns = [
+            (mpath.Path(np.full((1, 2), np.nan)), ls)
+            if ls == (0, None) else
+            (path, mlines._get_inverse_dash_pattern(*ls))
+            for (path, ls) in
+            zip(self._paths, itertools.cycle(self._linestyles))]
+
+        return zip(*path_patterns)
+
 
 class EventCollection(LineCollection):
     """

diff --git a/lib/matplotlib/lines.py b/lib/matplotlib/lines.py
@@ -60,6 +60,18 @@ def _get_dash_pattern(style):
     return offset, dashes
 
 
+def _get_inverse_dash_pattern(offset, dashes):
+    """Return the inverse of the given dash pattern, for filling the gaps."""
+    # Define the inverse pattern by moving the last gap to the start of the
+    # sequence.
+    gaps = dashes[-1:] + dashes[:-1]
+    # Set the offset so that this new first segment is skipped
+    # (see backend_bases.GraphicsContextBase.set_dashes for offset definition).
+    offset_gaps = offset + dashes[-1]
+
+    return offset_gaps, gaps
+
+
 def _scale_dashes(offset, dashes, lw):
     if not mpl.rcParams['lines.scale_dashes']:
         return offset, dashes
@@ -780,14 +792,8 @@ def draw(self, renderer):
                     lc_rgba = mcolors.to_rgba(self._gapcolor, self._alpha)
                     gc.set_foreground(lc_rgba, isRGBA=True)
 
-                    # Define the inverse pattern by moving the last gap to the
-                    # start of the sequence.
-                    dashes = self._dash_pattern[1]
-                    gaps = dashes[-1:] + dashes[:-1]
-                    # Set the offset so that this new first segment is skipped
-                    # (see backend_bases.GraphicsContextBase.set_dashes for
-                    # offset definition).
-                    offset_gaps = self._dash_pattern[0] + dashes[-1]
+                    offset_gaps, gaps = _get_inverse_dash_pattern(
+                        *self._dash_pattern)
 
                     gc.set_dashes(offset_gaps, gaps)
                     renderer.draw_path(gc, tpath, affine.frozen())

diff --git a/lib/matplotlib/tests/test_collections.py b/lib/matplotlib/tests/test_collections.py
@@ -1,5 +1,6 @@
 from datetime import datetime
 import io
+import itertools
 import re
 from types import SimpleNamespace
 
@@ -1191,3 +1192,27 @@ def test_check_offsets_dtype():
     unmasked_offsets = np.column_stack([x, y])
     scat.set_offsets(unmasked_offsets)
     assert isinstance(scat.get_offsets(), type(unmasked_offsets))
+
+
+@pytest.mark.parametrize('gapcolor', ['orange', ['r', 'k']])
+@check_figures_equal(extensions=['png'])
+@mpl.rc_context({'lines.linewidth': 20})
+def test_striped_lines(fig_test, fig_ref, gapcolor):
+    ax_test = fig_test.add_subplot(111)
+    ax_ref = fig_ref.add_subplot(111)
+
+    for ax in [ax_test, ax_ref]:
+        ax.set_xlim(0, 6)
+        ax.set_ylim(0, 1)
+
+    x = range(1, 6)
+    linestyles = [':', '-', '--']
+
+    ax_test.vlines(x, 0, 1, linestyle=linestyles, gapcolor=gapcolor, alpha=0.5)
+
+    if isinstance(gapcolor, str):
+        gapcolor = [gapcolor]
+
+    for x, gcol, ls in zip(x, itertools.cycle(gapcolor),
+                           itertools.cycle(linestyles)):
+        ax_ref.axvline(x, 0, 1, linestyle=ls, gapcolor=gcol, alpha=0.5)