Unfortunately, there is currently no simple way to create a gradient colored line in matplotlib. In this post, there are plenty of nice solutions to this problem, but they essentially all rely on using LineCollections (or pcolormesh, however such an artist is not supported by the legend). In and of itself, this is not a problem, but if you add the LineCollection to the legend, its key will only have a single color, instead of a gradient (e.g. using unutbu's solution as an example here):
Now I know you can create a custom handler, e.g. as explained here, but I have no idea how to do this for such a case. If I look at the source code of the HandlerLineCollection (which I thought would be an obvious starting point to figure out how to modify it) I noticed that it is based on Line2D, which cannot handle color gradients. So I don't know how to proceed from here.
How can I add a color gradient line to the legend, where its key shows (at least part of) the lines color gradient as well?
