Histogram Graph in ModelSim Simulator
AFAIK, the only graphing that Modelsim can do internally is to show the value of a multi-bit bus as an "analog" value. Therefore, one thing you could do is create a module for your testbench that reads out the histogram memory sequentially (either periodically or based on a trigger generated elsewhere in the testbench) and then display the output bus as a waveform.

The alternative would be to write the histogram data to an external file as it is created. An external script (Perl, Python, Ruby, etc.) that runs in parallel with your simulation would monitor that file for updates and then forward the data to something like Gnuplot to display it.