ChartGroup.BinsType (Word)

Specifies how the horizontal axis of the histogram chart is formatted, by bins type. Possible return values are xlBinsTypeAutomatic - Sets bins type automatically, xlBinsTypeBinCount - Sets bins type by count, xlBinsTypeBinSize - Sets bins type by size, xlBinsTypeCategorical - Sets bins type by category, xlBinsTypeManual - Sets bins type manually.

ActiveDocument.Background.Chart.ChartGroups(1).BinsType = xlBinsTypeAutomatic