Does anyone have an example using nonlinfit in Hypergraph?

I am trying to fit some data using an exponential decay function. I tried using polyfit and expfit, but neither really represented the data correctly. I tired nonlinfit but didn't get any results. It's possible I'm using the function incorrectly.

the example in documentation:  nonlinfit(x1, x2, y, {0, 0, 0}, "c1+c2*x1+c3*x2")

My attempt:  nonlinfit(p1w1c5.x,p1w1c5.y,0,0,"c1*c2^x1")

The message: "Invalid type of arguments in : Data 1....nonlinfit(p1w1c5.x,p1w1c5.y,0,0,"c1*c2^x1")."