2.1.1573 Part 1 Section, ST_BarGrouping (Bar Grouping)

a.   The standard states that the enumeration value standard specifies that the chart series are drawn next to each other on the depth axis.

In Office, the data markers for each category are drawn in clusters containing one marker from each series. The markers in each cluster are lined up on the depth axis in a 3D chart. On 2d charts, standard grouping behaves like clustered grouping.