Stackplots

I want to draw a [stackplot](https://matplotlib.org/stable/gallery/lines_bars_and_markers/stackplot_demo.html#sphx-glr-gallery-lines-bars-and-markers-stackplot-demo-py) in `Plot` widget, which may look like this:


But for filling there is only options to fill for convex polygon or under graph with fixed offset.

I think, to make this, there should be something like `Line::fill_between(graph_under)`.