Mpl is sending the number of vertex that are at least visible, for instance if the rectangle drawn is inside the bar it will send 0 paths since no paths are enclosed in that area. In the case of enclosing the right line it tries to create a mesh with the origin and the line enclosed. In the case of enclosing the left line since the origin and the line have the same y is just shown the line. This happens as well in Gtk and in none of the Agg backends neither backend_kivyagg. There is some clipping problem here.