Custom reports by category?

Ok, so I am trying to get the hang of custom reports. I am sure once I do, they will all be easy. But is there a spot in the forum where we can see custom reports that were already made? I was able to use the one for reporting daily sales in a month (broken down by day). All I am looking for now, is one that will do the month report by category. And use the categories I already have. I get that you can get a report with TAGS, but I just need a sales total by category… Once I get that, I think it will be easy to figure out reports by tag.

so far, I get how to make the rows and cells… I am still confused as to how to fill one side with CATEGORY and the total units sold and the other side with total sales from that category.

Sorry… I am an idiot. The normal reports show EXACTLY this.