That is an expansive question. I think you're going to have to come back with specifics of your setup, what you're trying to accomplish, and most importantly, what you've tried.
In general, you're going to need to:
1) Gather/Organize/Collect your data in a meaningful structure. This will involve Filter() to limit the results to a particular user for last year, as well as Sum() and maybe a few other formulas, based on your setup, tallying up the data in monthly chunks.
2) Add a chart to your app
3) Click into the chart and set the properties, including designating a Series to be your monthly tally