Allow chartsheets to contain multiple charts
Chartsheets can contain more than one chart. This is done in Excel by creating the charts and moving them to the chart sheets. Openpyxl can neither read nor create such chartsheets.
The implementation is interesting because it uses relative sizing. But it also relies on shoehorning the visible charts into the userShapes part of an essentially empty chart. Implementation might allow a simpler API for chartsheets: limit them to one chart only but introduce the idea of a multichart. NB. this is semantically very different to combi charts which share the same plot area.