ENH: enable striped lines

Author: arndRemy

doc/users/next_whats_new/striped_lines.rst

@@ -0,0 +1,19 @@
+Striped lines (experimental)
+----------------------------
+
+New *gapcolor* parameter enables the creation of striped lines.
+
+.. plot::
+    :include-source: true
+
+    import matplotlib.pyplot as plt
+    import numpy as np
+
+    x = np.linspace(1., 3., 10)
+    y = x**3
+
+    fig, ax = plt.subplots()
+    ax.plot(x, y, linestyle='--', color='orange', gapcolor='blue',
+            linewidth=3, label='a striped line')
+    ax.legend()
+    plt.show()

examples/lines_bars_and_markers/line_demo_dash_control.py

@@ -17,21 +17,33 @@
 :doc:`property_cycle </tutorials/intermediate/color_cycle>`
 by passing a list of dash sequences using the keyword *dashes* to the
 cycler. This is not shown within this example.
+
+Other attributes of the dash may also be set either with the relevant method
+(`~.Line2D.set_dash_capstyle`, `~.Line2D.set_dash_joinstyle`,
+`~.Line2D.set_gapcolor`) or by passing the property through a plotting
+function.
 """
 import numpy as np
 import matplotlib.pyplot as plt
 
 x = np.linspace(0, 10, 500)
 y = np.sin(x)
 
+plt.rc('lines', linewidth=2.5)
 fig, ax = plt.subplots()
 
-# Using set_dashes() to modify dashing of an existing line
-line1, = ax.plot(x, y, label='Using set_dashes()')
-line1.set_dashes([2, 2, 10, 2])  # 2pt line, 2pt break, 10pt line, 2pt break
+# Using set_dashes() and set_capstyle() to modify dashing of an existing line.
+line1, = ax.plot(x, y, label='Using set_dashes() and set_dash_capstyle()')
+line1.set_dashes([2, 2, 10, 2])  # 2pt line, 2pt break, 10pt line, 2pt break.
+line1.set_dash_capstyle('round')
 
-# Using plot(..., dashes=...) to set the dashing when creating a line
+# Using plot(..., dashes=...) to set the dashing when creating a line.
 line2, = ax.plot(x, y - 0.2, dashes=[6, 2], label='Using the dashes parameter')
 
-ax.legend()
+# Using plot(..., dashes=..., gapcolor=...) to set the dashing and
+# alternating color when creating a line.
+line3, = ax.plot(x, y - 0.4, dashes=[4, 4], gapcolor='tab:pink',
+                 label='Using the dashes and gapcolor parameters')
+
+ax.legend(handlelength=4)
 plt.show()

lib/matplotlib/lines.py

@@ -272,6 +272,7 @@ def __init__(self, xdata, ydata,
                  linewidth=None,  # all Nones default to rc
                  linestyle=None,
                  color=None,
+                 gapcolor=None,
                  marker=None,
                  markersize=None,
                  markeredgewidth=None,
@@ -364,6 +365,9 @@ def __init__(self, xdata, ydata,
         else:
             self._marker = marker
 
+        self._gapcolor = None
+        self.set_gapcolor(gapcolor)
+
         self._markevery = None
         self._markersize = None
         self._antialiased = None
@@ -754,9 +758,6 @@ def draw(self, renderer):
                 self._set_gc_clip(gc)
                 gc.set_url(self.get_url())
 
-                lc_rgba = mcolors.to_rgba(self._color, self._alpha)
-                gc.set_foreground(lc_rgba, isRGBA=True)
-
                 gc.set_antialiased(self._antialiased)
                 gc.set_linewidth(self._linewidth)
 
@@ -772,6 +773,26 @@ def draw(self, renderer):
                 if self.get_sketch_params() is not None:
                     gc.set_sketch_params(*self.get_sketch_params())
 
+                # We first draw a path within the gaps if needed.
+                if self.is_dashed() and self._gapcolor is not None:
+                    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]
+
+                    gc.set_dashes(offset_gaps, gaps)
+                    renderer.draw_path(gc, tpath, affine.frozen())
+
+                lc_rgba = mcolors.to_rgba(self._color, self._alpha)
+                gc.set_foreground(lc_rgba, isRGBA=True)
+
                 gc.set_dashes(*self._dash_pattern)
                 renderer.draw_path(gc, tpath, affine.frozen())
                 gc.restore()
@@ -876,6 +897,14 @@ def get_drawstyle(self):
         """
         return self._drawstyle
 
+    def get_gapcolor(self):
+        """
+        Return the line gapcolor.
+
+        See also `~.Line2D.set_gapcolor`.
+        """
+        return self._gapcolor
+
     def get_linestyle(self):
         """
         Return the linestyle.
@@ -1066,6 +1095,29 @@ def set_drawstyle(self, drawstyle):
             self._invalidx = True
         self._drawstyle = drawstyle
 
+    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 None
+            The color with which to fill the gaps. If None, the gaps are
+            unfilled.
+        """
+        if gapcolor is not None:
+            mcolors._check_color_like(color=gapcolor)
+        self._gapcolor = gapcolor
+        self.stale = True
+
     def set_linewidth(self, w):
         """
         Set the line width in points.
@@ -1247,6 +1299,9 @@ def set_dashes(self, seq):
         For example, (5, 2, 1, 2) describes a sequence of 5 point and 1 point
         dashes separated by 2 point spaces.
 
+        See also `~.Line2D.set_gapcolor`, which allows those spaces to be
+        filled with a color.
+
         Parameters
         ----------
         seq : sequence of floats (on/off ink in points) or (None, None)
@@ -1264,6 +1319,7 @@ def update_from(self, other):
         self._linestyle = other._linestyle
         self._linewidth = other._linewidth
         self._color = other._color
+        self._gapcolor = other._gapcolor
         self._markersize = other._markersize
         self._markerfacecolor = other._markerfacecolor
         self._markerfacecoloralt = other._markerfacecoloralt

lib/matplotlib/tests/baseline_images/test_lines/striped_line.png

@@ -304,6 +304,17 @@ def test_marker_as_markerstyle():
     assert_array_equal(line3.get_marker().vertices, triangle1.vertices)
 
 
+@image_comparison(['striped_line.png'], remove_text=True, style='mpl20')
+def test_striped_lines():
+    rng = np.random.default_rng(19680801)
+    _, ax = plt.subplots()
+    ax.plot(rng.uniform(size=12), color='orange', gapcolor='blue',
+            linestyle='--', lw=5, label=' ')
+    ax.plot(rng.uniform(size=12), color='red', gapcolor='black',
+            linestyle=(0, (2, 5, 4, 2)), lw=5, label=' ', alpha=0.5)
+    ax.legend(handlelength=5)
+
+
 @check_figures_equal()
 def test_odd_dashes(fig_test, fig_ref):
     fig_test.add_subplot().plot([1, 2], dashes=[1, 2, 3])