This is a quirk in the way the legacy charts work for categorical charts.
X-Axis works fine for date-formats, provided you set either a date or a number format. If you set a number format, then this will take precedence over the date-format. So make sure that if you set the date format, you clear the "numeric-format" property first.
There is nothing I can fix without breaking all existing reports that use these format strings. The thing has been ill-defined in the past.
The property "x-axis-label-rotation" works as well.
There is a second property called "label-rotation" along with the "show-labels" property. This one controls the value-labels that are printed on the data-items and it works once the "show-labels" property is set to "show labels". These two properties make no sense in the "X-axis grouping". (
PRD-3461 asks for moving these properties away from the x-axis).