ChartFont.Background (PowerPoint)

Returns or sets the type of background for text used in charts. Read/write Variant that is set to one of the constants of XlBackground.

Possible Values are xlBackgroundAutomatic - Word controls the background, xlBackgroundOpaque - An opaque background, xlBackgroundTransparent - A transparent background.
ActivePresentation.Slides(1).Shapes(1).Chart.DataTable.Font.Background = xlBackgroundAutomatic