I suspect there are several ways to do this. The key for me was being able to use the mouse coordinates to indicate a specific item in a grid. I used a table I created within the composition for listing the specific items, which are the positions in the list. You could alternately input a table using a Fetch Table node, and output whatever IDs you need for your lights.
I’m not familiar with Art-Net, but our example compositions show how to send and received Art-Net messages. (This node set is included in Vuo Pro. If not using Vuo Pro, then you would need another protocol to communicate with your equipment.)
This should be a starting point. It will hard-cut between intensity changes, and as it stands it should go to black between the boxes. To change this, set the “Is Within Box” width and height values to 0,25. You can get the “Make 2d Point Grid” node here: https://community.vuo.org/t/-/6237.