I use the grlib demo code from the Starterware, why calculate the X&Y coordinate from the ADC value as follows?
xdata = (((xdata * 2130) - (xdata * 5))/10000) - 53;
ydata = (((xdata * 5) + (ydata * 1321))/10000) - 29;
And where is the origin? the Left-top corner??