ShowAsKind:
| "percentOfTotal"
| "percentOfRow"
| "percentOfCol"
| "index"
| "percentOfParentRow"
| "percentOfParentCol"
| "percentOfParent"
| "runTotal"
| "percentOfRunningTotal"
| "rankAscending"
| "rankDescending"
| "difference"
| "percent"
| "percentDiff"
How a value can be shown, as OOXML spells it.