Since Graphmatica allows you to superimpose one graph on top of another indefinitely, after a while the grid may be a huge mess. At this point you might want to select Clear Screen in the View menu to clear the screen. Everything will be erased and the grid will then be redrawn. By default, up to 25 equations you have typed are still stored and can be redrawn even after you have cleared the screen. In fact, every graph that was on the screen prior to clearing can be recalled instantly and will not even need to be recalculated until the grid size changes.
You can also clear the screen using the Clear button on the toolbar.