Office occupation % heat map

Hi everyone,

I'm looking to make a heat map that indicates when a desk at an office is empty or used.

I have a dataset such as the following:

For example the first line shows someone sat on desk 0001 at 11:34:45 on the 11th, and the second line shows that he left the desk at 18:23:32 the same day.

What I am trying to get from this data is a heat map that shows office % occupation for every 15 minutes interval between 09:00 and 20:00.

Should I build a database based on this one that gives the state of a desk for each interval (which would mean more than 10 million lines in my dataset), or is there a way to create a heat map from this data alone, that could for each 15 minute interval look at the last known state and attribute it correctly ?

Thank you very much for you time.