FFT MegaCore simulation

You're not very clear on what you're doing. My guess is you're using DSP Builder and the FFT Megacore as that screenshot looks like a Simulink scope. If you want useful answers you need to give much more information so that people don't have to guess.

You've connected your input data and output data to a scope, which always shows a signal plotted against time. To get a graph of the frequency you need to save that data to a variable using a "To Workspace" block. Then you can use the MATLAB command "plot" the points against any values you like.

You may be able to do it directly using the vector scope block as well.