I think you're asking for a totals column -- not a column in the graph, but
a column of text showing the totals.

The datatable options doesn't have any flexibility. I think you'll have to
do this manually.

This is what I did, if it helps.  We had a column chart showing totals of
individual departments.  However, they also wanted to show the totals, but
not in the column chart.  I did the original numbers in Excel and selected
only the department numbers for the column chart.  For the slide itself, I
pasted the column chart in, and then I copied the totals from the spreadsheet
and pasted that into PowerPoint, too.  In my case, I linked the files so when
they made changes, the chart would automatically update.