The GridLine command enables a grid of colored lines on the graph screen (you can disable it with the GridOff command). How fine or coarse the grid is depends on the Xscl and Yscl variables. Drawing the grid just involves plotting points all the horizontal and vertical lines that intersect points of the form (A×Xscl, B×Yscl) that are in the graphing window. The grid can be colored based on any color variable or value.
GridLine can be used to shade the entire screen if Yscl is small enough that the lines on the grid are one pixel apart:
:ΔY→Yscl :GridLine RED
This is one of the shortest ways to shade the screen, although Shade( can be used for a (usually) even shorter way. However, using GridLine is also very slow: the fastest way involves the Horizontal or the Vertical commands in a For( loop.
You could also use GridLine to draw the playing grid for a Dots and Boxes game.