Accessing error matrix

Hello everyone,
I must apologize in advance if I’m missing something obvious but I’ve done some searching around the Users guide and google and have been unable to find what I’m looking for. I’m trying to set up an energy calibration using 4 points with errors in both bin number and energy. I put the points and errors into a graph and use the “pol1” fit which works fine. Is there a function that will extrapolate errors in energy for an arbitrary bin# using the fit that I created? I’ve also been trying to access the error matrix in an attempt to do it myself with no luck.

If you have a TF1 *myfunc object, the result of your fit, you can call