Need a horizontal StackedBarChart

So far we can only create vertical StackedBarChart. I believe it won't be hard to flip the coordinates to make horizontal ones.

Start by locating the existing StackedBarChart implementation and tracing how it calculates vertical coordinates. Determine how horizontal orientation should be represented, then verify that the chart renders stacked values left-to-right with the expected labels and proportions. The issue does not name a test or file, so inspect existing chart tests before adding coverage.

Written by the indexing model from the issue text.