Each category has two parts, a_values and b_values. Rescale each category so its two parts sum to 1.0, then draw them stacked, A at the base and B on top. Every bar therefore reaches exactly 1.0 regardless of the raw totals, which is the point: the chart compares composition, not size.

Bars sit at positions 0, 1, 2, .... Label the series "A" and "B" and show the legend. Return the Axes.