Horizontal bar supporting stacked graphs

Currently you can have a stacked bar graph vertically, but when you try it using the horizontal bar you just end up with multiple items in the chart rather than it stacking.
Any way to have a stacked horizontal bar graph?

Start by reproducing the difference between vertical stacked bars and horizontal bars in pygal. Locate the chart entry points and existing stacking behavior, then define completion as horizontal bar charts rendering stacked series as one combined bar rather than separate items.

Written by the indexing model from the issue text.