A circle and square are just two shapes I used to indicate entrances and exits.
That is not a real chart of anything - I made it up in 3 minutes in turbocad, because that is what I had open at the time - square and circle are the only two shapes that only require two clicks to draw on the grid. I have done proper ones before, where the line extends before the entrance for things like getting actors into special locations (ie inside a prop piece), so I have always indicated the moment the actor actually appears as well as the time when they are 'needed'. As for labels, names written down the side, and times down the bottom. I divide by act/scene/10 - so 10 cells within a scene (so I can easily offset the actors entrances). If I need to write anything, I would just put text over the line (ie if an actor plays multiple characters, I may put a label over the line).
You could just use excel and shade rows of cells, whatever works for you.