How to make a graph to display the monthly growth percentage of a metric

I have a metric which gives me the Disk size of my data system. I would like to know the value of Size growth of the data system for each month in percentage. For eg: During June month it is 4 GB and during July it may be 4.5 GB. So 0.5 GB has been increased for this month. I need that data to be find and display as growth percentage. Is it possible to make use of Virtual datapoint in this case?

Can I get any suggestions to achieve this please.