A combo chart (columns plus lines, often with the lines on a second value
axis) was read with each series' type but drawn as lines only, on one
scale, so a spend-and-clicks chart showed the clicks as a flat line along
the bottom and no columns.
- Reading: a plot group whose value axis isn't the first group's is on the
secondary axis (Series::secondary_axis, now set).
- Model: Chart::series_kind, on_secondary and axis_range (per axis; a combo
stacks only its stacked series).
- Rendering: bar-like series are drawn as bars (clustered or stacked among
themselves), the others as lines/areas over them, each against its own
axis; the secondary axis' labels go on the right. Line series use their
line's colour and width (where PowerPoint keeps them), also in the legend.
Long category labels wrap onto two lines.
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