The scatter viewer currently has a bug (noticed by @victoriaono) where the line collection that connects points will only obey the selected colormap if that colormap was set before a line was first displayed on the viewer. Additionally, if a fixed color is set after the line displays, subsequent colormap usage will not be respected. See the video below for an example.

This PR creates consistent behavior by calling set_color(None) inside the line collection's set_linearcolor method. The reason for this has to do with how the behavior is implemented inside matplotlib. Whenever the colormap mode is Fixed, we call set_linearcolor with color=self.state.color here. This call's the artist's set_color method, which ultimately calls set_facecolor and sets _original_facecolor to that color.

However, when cmap_mode is set to Linear, we call set_mpl_artist_cmap here without a color argument, which doesn't doesn't call set_color, and so _original_facecolor remains as the solid color. The artist's _edge_is_mapped attribute is set to false if _original_edgecolor has a color value, which means that the edgecolors of the artist are set to the original edgecolor, rather than the colormapped colors. This is why things work if we set the colormap before the line first displays - we haven't made a call to the line collection's set_color method yet, so _original_facecolor is still None. By calling set_color(None), we reset _original_facecolor to None, allowing the line collection edges to be colormapped.