Create year over year trend graph

Best to do this on a % full basis, to give an idea of how much of the parking infrastructure is being utilized and where that's heading (is it being use more or less?)

This brings up amount of data issues, is what we have all of 2011 and all of 2012 (2012 has much more data), and we would need 2013 to have a good idea of a trend.

perhaps do this on a month-by-month basis first.
