Create a churn percentage chart

Create a churn percentage chart

I'm looking to create a chart that shows our monthly churn percentage. I've attached two charts I'm able to create, one showing a running total of contact records - the other, showing monthly cancellations. 

We need to see the percentage of cancellations-to-running total for each month. For example, looking at the two attached graphs: 

Jan 2014 there are 16 cancellations, and 430 total contacts, so the churn rate would be 16/430 or 3.7% 

In Dec, there were 9 cancelations and 395 total contacts, so the churn would be 2.3% 

Does anyone know how to create an ongoing graph that would show this moving churn rate?

Thank you!