How do I extract the gradient at a specified location?

Hello,

I'm new to COMSOL. I tried to use COMSOL to solve Fick's second law, Ct=Cxx, for a 1-D geometry using the coefficient form PDE. Once I have C, which is a function of x and t, I'd like to plot the gradient of C at x=0 as a function of time. Is there a way to specify Cx at x=0?

Thanks,
Dai