Graphing gym membership over time

Hello,

I have a spreadsheet with data on when certain people joined a gym, looking something like this:
October 2011
May 2012
December 2012
October 2011
Fabruary 2015

etc.
I want to make a graph that shows accumulated gym membership over time (assuming no one cancelled their membership). How can I do this?

Take you very much for your time.
Best,
Nora