Why do you want to do this?

Does that matter? To be clear, I almost never use scatter. I just included it to demonstrate that the method works for one but not the other. I often, however, use plot to plot an experimental response and then plot again to make a marker that points out a significant moment.

The reason for this is a bit obscure. There are actually 2 artists (and in general N) and some reason only the first artist makes it into the legendHandles (probably to preserve back-compatibility when the ability to have compound artists was added ?). If you plot a line with a linestyle you will see it change color (but not the marker). The full set of artists is only accessible via a deeply nested set of private attributes that manage the layout of the legend.

To control exactly what is in the legend I suggest having a look at https://matplotlib.org/users/legend_guide.html#creating-artists-specifically-for-adding-to-the-legend-aka-proxy-artists Doing it this way you will also be able to hold onto a reference to the handles and you can change them however you wish.

Thanks. Indeed, I was thinking about this and came up with several ways to demonstrate the complicated behavior of markers in a legend. Surely you can at least imagine why someone might want to increase the handle linewidth, or analogously, the marker size.

import matplotlib.pyplot as plt
plt.plot([0,1],[0,0],'r', label='LH[0]')
plt.plot([0,.4],[-.1,-.1],'go', label='LH[1]')
plt.plot([0,.4],[-.2,-.2],'k^', label='LH[2]') 
plt.plot([0,.4],[-.3,-.3],'bs', label='LH[3]') 
plt.plot([0,.4],[-.4,-.4],'D', mfc='c', label='LH[4]')
plt.scatter([0,.4],[-.5,-.5],c='m', label='LH[5]')
leg = plt.legend(loc='right', labelspacing=2)
LH = leg.legendHandles
LH[0].set_linewidth(8)  # Works
LH[1].set_color('y') # No change
LH[2].set_ms(15)    # No change
LH[3].set_marker('*')  # Stars added to left and right!
LH[3].set_color('r')   # Stars now change color
LH[4].set_color('w')   # No change
LH[5].set_alpha(0.5)  # Well-behaved scatter

Right, the artist in legendHandles is the 'line' part of the legend handle (not the marker), the changes are having an effect on the internal state, the line is just draw with out a line style and without markers.

If you want complete control of the legend, use the proxy artist path.

Unfortunately, to expose more of the automatic legend internals would require a pretty big over-haul of the legend generation code.

This is a very long standing issue (for example, see #2532 ).

I encoutered this issue recently.
My dirty-fix was provided to me by @tomMoral:

ax_legend = fig.add_subplot(g[3, 0])
ax_legend.axis('off')
handles_markers = []
markers_labels = []
for marker_name, marker_style in markers_style.items():
    pts = plt.scatter([0], [0], marker=marker_style, c='black', label=marker_name)
    handles_markers.append(pts)
    markers_labels.append(marker_name)
    pts.remove()
ax_legend.legend(handles_markers, markers_labels, loc='center', ncol=len(markers_labels), handlelength=1.5, handletextpad=.1)

where markers_style would be a dict mapping your markers' labels to their shape.