I could use your advice on a UX issue. I'm working on the UX of a system for scheduling appointments. Showing something is either available or not available is easy, but I want to give people the ability to add themselves to a wait list, should the appointment they want come available. My problem is I'm not sure the best way to indicate something isn't available. Before the add to wait list functionality was needed, I just left the grid blank where an appointment was booked. Now I need to add in a booked, but can be wait listed. Any ideas?