ChartFont.Underline (PowerPoint)

Returns or sets the type of underline applied to the font. Can be one of the XlUnderlineStyle constants. Read/write Variant.

Possible Values are xlUnderlineStyleDouble - Double thick underline, xlUnderlineStyleDoubleAccounting - Two thin underlines placed close together, xlUnderlineStyleNone - No underline, xlUnderlineStyleSingle - Single underline, xlUnderlineStyleSingleAccounting - Not supported.
ActivePresentation.Slides(1).Shapes(1).Chart.DataTable.Font.Underline = xlUnderlineStyleDouble