Add marker-only and line+marker visuals to the plot() plot types

Inspired from the discussion in matplotlib#27765: We should visually communicate
that `plot()` covers all three variants: markers only,
line+markers, line-only.
They are visually distinct enough that it's not possible
to infer the variants if you see only one.
In particular, it's important to communicate that you
can draw markers only. We don't want to automatically drive people who
want markers (e.g. some discrete measurements of a dependent variable y
(x)) to scatter because that's the only one showing
 discrete markers in the overview.

Author: timhoffm

galleries/plot_types/basic/plot.py

@@ -13,12 +13,16 @@
 
 # make data
 x = np.linspace(0, 10, 100)
-y = 4 + 2 * np.sin(2 * x)
+y = 4 + 1 * np.sin(2 * x)
+x2 = np.linspace(0, 10, 25)
+y2 = 4 + 1 * np.sin(2 * x2)
 
 # plot
 fig, ax = plt.subplots()
 
+ax.plot(x2, y2 + 2.5, 'x', markeredgewidth=2)
 ax.plot(x, y, linewidth=2.0)
+ax.plot(x2, y2 - 2.5, 'o-', linewidth=2)
 
 ax.set(xlim=(0, 8), xticks=np.arange(1, 8),
        ylim=(0, 8), yticks=np.arange(1, 8))