Hi,
If I understand you correctly you just want to change how the value is displayed? You use ISE for simulation? AFAIR ISE is not able to display analog signals as other simulators can. On thing you could do is to write the values to a file and use another tool (matlab, gnuplot, ...) to plot the waveform.
It has been a while that I have used the ISE simulator, so you could check whether they have incorporated the feature in the meantime. If you click right on the signal, there should be a way to choose the radix (a list like: Decimal (Signed), Decimal (Unsigned), Binary, Hexadecimal, ASCII).
If there is an analog option, use it, otherwise ISE does not support it.
If you are using ISE/modelsim than you should be able to simply change the radix of the signal to analog (via the GUI or the modelsim console ("radix analog")).