If anyone knows any way to get rid of the undefined placeholder and make it just empty, feel free to show me and/or link a graph to it! If you find a better way to do this also tell me.

Here’s a different way. Basically taking the list n, but only keeping those elements with remainder, 0. Then, down below, using L_2 instead of Y for the purple points. You could easily make a second list if you wanted another set of points a different color. L_3= n [mod(n,d)>0]