Have a look at the stat you imported on the Statistics tab (make sure you select the correct Section) and see what the expression is - the rakeback multiplier is probably in the stat's Value Expression, but depending on how it is built it might be in the column used by the stat. If it is done in the column see which expression is used by the stat and then go to the columns tab and look up that column.
Tutorial: Custom Reports and Statistics
Whether you can use the rakeback stat in the graph depends on what section it is in?
It would need to be in the Holdem Cash Player Statistics section to be used in the graph, but it won't show you a running total, only the rakeback earned in each session.