Ah, not so much that, although not a bad idea, more that I don't have an external GPS and can't get a signal through the windows.
I was thinking of an older program which had the ability to manually enter your position and update it as required.
You could also enter some variables you knew...such as IAS, groundspeed, track, HDG. and could then plot your track based on what info you had provided.
Obviously the plot is only so good for as long as you update the info...
...but should you have a valid GPS signal, then adding those variables you know from the aircraft can be useful for working out things such as your wind (headwind/tailwind) etc. (Bit like a garmin pilot GPS)