It's not possible, the XY Graph only accepts scalar inputs, which means that you can only plot one x-y graph at a time.
I would suggest storing the relevant data to the MATLAB workspace using To Workspace blocks, writing a MATLAB function or script that gets executed when the model completes its execution through the StopFcn callback (see Using Callback Functions in the Simulink documentation for more details). Then you can use the hold on in your MATLAB code command as suggested above.
HTH,
Arnaud
