Calculation for Growth YOY

Hey!

I need help figuring out a calculation for growth YOY. I have a dataflow that streams in total sales, which I created a bar graph for YOY comparison for each month. From my data set, I am pulling in from a third party, each line has a date and amount in sales. I want to see growth from Jan 2018 to Jan 2017 as well as Jan 2017 to Jan 2016 & etc......

I have a good visual for total sales using the period over period bar graph. Then, I compare it to 1 + 2 + 3 years of data. 

Can anyone help me out with this with the calcuation for growth?

Thanks! 

Thanks!