Spreadsheet contains records of apointments that have taken place, including
start and end times of appointments e.g. 12:30 pm to 17:30 pm.

Need to produce a chart that has "number of appointments" on y-axis and 15
minute timebands on x-axis. Idea is to visually demonstrate periods of high
and low activity.

Problem I'm having is working out how many appointment instances occur for
each 15 minute timeband.