The grid shows gaps between lines, as demonstrated on the image:

These gaps can vary, depending on the "steps" on the addon control slider. Unless that's intended?

Addon Version
9.0.23-Retail

World of Warcraft Version
Retail

Its not intentional, but is kindof a weird artifact of how the wow ui draws single pixel lines at various screen resolution + UI scale settings.

This should be fixed in the latest version. I'm using a two pixel line with as @Goranaws was earlier, as well as some PixelUtil stuff that Blizzard has built into the retail client (that will eventually make it into the classic clients)