ΔX records the change in X for each pixel. So, if each pixel represented one unit, this would be set to 1. ΔX is updated every time Xmin or Xmax is changed, or when Full, Horiz and G-T is called. When this is changes, Xmin remains the same, but Xmax is changes so that ΔX is the change is X for each pixel. For a TI-84 calculator, this will be equal to (Xmax-Xmin)/94 in Full or Horiz mode, and equal to (Xmax-Xmin)/47 is G-T mode since a TI-84 is 94 pixels wide.