okay, but what kind of running total? manual running total, running total expert?
if the former then, you can 'roll up' the values and pass them to a subreport to chart off of. please see this blog post.
if the latter (i.e. using the running total expert), then you should be able to add this directly to the chart. the running total expert can be handy for eliminating duplicates as it has an "Evaluate" option that can iterate the running total based on a change in a field.