As the table is the product of sub-queries I need the rows for each calculation but my target is the summary.
As far as I've been able to figure, it is not possible to do this. I was initially thinking that some conditional formatting trickery but some further investigation curtailed this train of thought pretty quickly.
We tried to find a cheeky workaround a couple of ways, but we weren't able to achieve exactly what you are looking for. Depending on how your report is set up, it might be possible to massage things a couple of ways though.
One way to come somewhat close could be to use Grand Totals in your Data builder, and then set Numeric Display as your chart type. Your important value will be the only thing that shows up in the chart. Although likely not ideal, it is a way of "hiding" the rows that you don't need displayed.
Another way to ignore all of the non-grand total values would be to set up a Report Summary. Details on how to do that can be found here:
Sorry to be the bearer of less than good news on this one. Please let us know if you have any further questions.