Chart sheets (a sheet that holds one chart; Move Chart > New sheet, F11) were
skipped on open, and their chart sheet, drawing, chart, style and colour parts
were dropped with them because the pass-through treats those kinds as content
GridCraft rewrites. Saving a workbook lost every chart sheet.
They are now kept as they were: the <sheet> entry (name, visibility and its
place among the worksheets), the chartsheet part and everything behind it,
byte for byte, written back in place. The workbook's other sheet positions
(active tab, print areas, scoped names) are computed with the chart sheets
counted. A chart sheet is left out on save when the chart is no longer
valid for the workbook: a sheet it plots was deleted or renamed, or a
worksheet took its name.
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