Convert named_colors example to use Rectangle (#16467)

* Convert named_colors example to use Rectangle

In line with #15614, this changes the named_colors example in the docs
to use a Rectangle rather than drawing lines. This should make it more
flexible to update in future.

* Remove stating default behaviour of fill in example

Co-Authored-By: Tim Hoffmann <2836374+timhoffm@users.noreply.github.com>

Co-authored-by: Tim Hoffmann <2836374+timhoffm@users.noreply.github.com>

examples/color/named_colors.py

@@ -14,6 +14,7 @@
 * the :doc:`/gallery/color/color_demo`.
 """
 
+from matplotlib.patches import Rectangle
 import matplotlib.pyplot as plt
 import matplotlib.colors as mcolors
 
@@ -59,15 +60,16 @@ def plot_colortable(colors, title, sort_colors=True, emptycols=0):
         y = row * cell_height
 
         swatch_start_x = cell_width * col
-        swatch_end_x = cell_width * col + swatch_width
         text_pos_x = cell_width * col + swatch_width + 7
 
         ax.text(text_pos_x, y, name, fontsize=14,
                 horizontalalignment='left',
                 verticalalignment='center')
 
-        ax.hlines(y, swatch_start_x, swatch_end_x,
-                  color=colors[name], linewidth=18)
+        ax.add_patch(
+            Rectangle(xy=(swatch_start_x, y-9), width=swatch_width,
+                      height=18, facecolor=colors[name])
+        )
 
     return fig
 
@@ -103,4 +105,4 @@ def plot_colortable(colors, title, sort_colors=True, emptycols=0):
 matplotlib.figure.Figure.get_size_inches
 matplotlib.figure.Figure.subplots_adjust
 matplotlib.axes.Axes.text
-matplotlib.axes.Axes.hlines
+matplotlib.patches.Rectangle