Hi David
I am not sure if we can suppress Lines in Graph when certain conditions are not met.
However, you can create a sub-report and display the chart in it applying the filter in record selection of Sub report as Value>0. Also if you need that data always greater than 0 in main report as well you can apply the same in report as well so that you don't have to worry about suppressing that in Chart.
Let me know if this helps.
Regards
Niraj
