YOu could do two things. First, use a subreport for the chart that shows all the data plotted. Secondly, use a conditional formula that hides any record after the fifteenth one printed. That way the data is still there for CR to chart it, but it will be hidden from the report.
I cover a multitude of examples for using formulas in your report in Chapers 5-7 in my Encyclopedia book. You can find out more about my books at
Amazon.com
or reading the
Crystal
Reports eBooks online.