there are so many things to do it better than with that freaky skope block.
Use the Wavescope from the Xilinx Blockset. It can handle multiple signals with different radices and has even a semi-analog display option. All in one window.
If you have Modelsim anyway and are using Black-Boxes with some HDL, then you can view these directly in the Modelsim waveform window, provided that you choose the option to leave the Waveform window opened after simulation. But I think that's the default setting. However, yiou need to add the Modelsim Block to your Design, and the Black Boxes need to be configured to use Modelsim as their external HDL-Simulation tool.