Bar Charts: Simple, Grouped & Stacked

The problem, from first principles

A bar chart compares quantities across categories, and it is the chart most often drawn wrongly. Matplotlib does not know that your x values are categories: it places each bar at a numeric position (0, 1, 2, ...) and you then tell it what text to print under each position. A grouped chart puts several bars side by side inside one category, so each bar's position has to be shifted by hand. A stacked chart puts bars on top of each other, so each bar needs to know where the one below it ends.

The geometry is simple but unforgiving, and since every bar is a Rectangle with an x, a width, a y (its bottom) and a height, you can check every one of them. This question builds the three charts and the labelling step that puts each value on top of its bar.

From theory to code

Implement bar_chart(labels, values), grouped_bars(labels, series), stacked_bars(labels, series) and annotate_bars(ax). The signatures and docstrings are already in the editor.

Constraints

• labels is a list of n category names; values and each list in series have n non-negative numbers. series is a dict mapping a series name to its list of n values; iterate it in insertion order.
• bar_chart(labels, values) returns (fig, ax): bars centred at positions 0..n-1, each of width 0.8 (the default), x tick positions 0..n-1 labelled with labels, and the bottom of the y axis at 0.
• grouped_bars(labels, series) returns (fig, ax). With k series, each bar has width 0.8 / k. Bar j of group i is centred at i + (j - (k - 1) / 2) * width. Ticks are at 0..n-1 labelled with labels. The legend lists the series names.
• stacked_bars(labels, series) returns (fig, ax). Every series draws bars of width 0.6 centred at 0..n-1; the bar of series j starts (y) where the bars of the series before it end. Ticks and legend as above.
• annotate_bars(ax) writes the height of every bar (ax.patches) as text, formatted with f"{height:g}", centred horizontally above the top of that bar (y + height), and returns the list of those strings in the order of ax.patches.

Hints