I am sorry but I have not understood your question and it is not clear to me what your plot represents.
What are the axes ( I guess the parameters), but what is plotted ? And what the color rapresents ?
Please explain,
You can get the parameter contours which defines their confidence regions from Minuit, see the tutorial
fit/fitcont.C root.cern.ch/doc/master/fitcont_8C.html