Histogram, box & whisker, waterfall, funnel, treemap and sunburst charts
were written as clustered columns, and reading skipped such charts in
files made by Excel, which keeps them as cx:chartSpace (chartex) parts.
The writer now writes these kinds as chartEx parts, as Excel lays them
out: the data goes through hidden _xlchart.v1.N workbook names, each
part comes with chart style and colour parts, and the drawing frames it
in mc:AlternateContent with a text-box fallback. Fills are written out
(series in theme accents, waterfall decreases and treemap/sunburst boxes
in their own, a white chart area), so they don't depend on the style
part. The reader reads the parts back (resolving the _xlchart names,
which aren't kept as names) with data, title, axis titles, legend,
labels and colours, and follows mc:AlternateContent in drawings.
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