Chart.SeriesNameLevel (Word)

Returns or sets an XlSeriesNameLevel constant that specifies the source level of the series names. Possible return values are xlSeriesNameLevelAll - Sets series names to all series name levels within range on the chart. The default, xlSeriesNameLevelCustom - Indicates literal data in the series names, xlSeriesNameLevelNone - Sets no series names in the chart.

ActiveDocument.Background.Chart.SeriesNameLevel = xlSeriesNameLevelAll