Count of active customers line graph

Hello,

I have a dataset of customers which contains a start date and an end date. From these I have a calculated colum for both the start business year and the end business year. The business year is a text field and has values of the format 2013-14 for example. The business year runs from August to July.

I would welcome suggestions of how to plot the count of active customers against business year as a line graph.

i.e. a customer is considered active when: start business year <= business year <= end business year.

Thanks.