Plotly's own ridgeline recipe: horizontal violin traces with
side: 'positive', widened so the halved curves overlap. MAIDR
emits one ridgeline layer rather than the violin pair, because the recipe
draws no inner box for a quartile summary to describe. Every group is
pitched against the tallest density anywhere on the chart, so the ridges
can be compared — and moving between groups holds the value the reader is
on rather than the column index, since the curves need not share a grid.