How can I do a comparison in a pivot chart

Hi there.

I capture transaction values in a table record and then assign it to a category in the same record called purchase. That means I can easily calculate the average transaction by dividing the record count by the transaction values.

However, what if I wanted to create a pivot table that showed the average transaction cost year vs the average transaction cost for the year before, and then wanted to show a third column in the table where it showed the change.

Example:
This year: 10
Last Year: 100
Change: -163%

Struggling to get my head around this.

Thanks