Graph with cumulated values over period of time

Hi,

I have records of customer presence with a start and end date on each row like:

"Joe", "12-Dec-15", "23-Jan-16"

"Mary", "20-Dec-15", "12-Jan-16"

Etc...

I can only produce a graph that shows either the start or end on the axis. However, I need to have a graph over time that counts these customers over the span time betwen start and end. 

How can I achieve that? Thanks.