Find equation from spline curve graph [SigmaPlot]

Hi everyone, I am using SigmaPlot to build cumulative density functions to look at the percent arrival of insects over a 2 month period for several years. I have attached a sample graph for a visual. I know that the splines to fit these curves have to come from an equation but I do not know how to find out what equation SigmaPlot is using to fit them. I just used the graph builder, I did not give it parameter estimates explicitly.

I normally use R for analyses and performed a glm on percent arrival but my estimations from the results do not match up with the graph. Does anyone know if there is a way to find the equation in SigmaPlot so I can see how the program fits the curve? The curve for 2009 dips below 0, and goes above 100, which is, obviously, not possible in this situation so I want to know what I am missing here....Thanks!