how to show excluded filtered data?

I have a list of data of rooms and times. So room411 might be in one cell and time 2:00pm - 4:00pm will be in cell next to it. I can use a filter to see all the rooms in use at different times but I can't do the reverse and find the rooms not in use at a specific time. Is there anyway to do that? Or to plot a timeline chart showing when its in use and when its not?