There's maybe a workaround here. If you can create a straight table in a hidden sheet that returns the correct number of rows to answer your question then this macro:
ActiveDocument.Variables("vRowCount").SetContent chart.GetNoOfRows, true
Replace CH01 with the name of your chart.
Will put the number of rows into the variable vRowCount.
Of course you'll need to click a button or such like to run the macro and update the value of the variable if selections change.