ΔX records the change in X for each pixel. So, if each pixel represented one unit, this would be set to 1.
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.