Assume it's a 100% stacked column?
Add another series with the monthly totals, which starts life as another
stacked column. Select this series, go to Chart menu > Chart Type, and
change it to a line type, so it doesn't mess up the columns. Double click on
the line series, and on the patterns tab, format it to be invisible: no
lines, no markers.
If it's a 100% stacked line, you can still add the totals, convert it to a
column, and hide the column. The difference here is that the total appears
above the other series in the list, and they are no longer listed in the
same order as they appear in the chart (the highest in the stack is listed
lowest in the table). These things cannot be changed.
I hardly ever use data tables in the chart. Aside from the inflexibility in
their order, you also have little leeway in formatting of the tables, and
you are limited as to what data to show. I make a table in a worksheet range
near the chart. Even though it doesn't line up perfectly, it's a lot better
display than a chart's data table.
- Jon