When it comes to your financial data, you should use the Financial Report under "General Stats" that is below the sales graph. While the sales graph is a good visualization of your sales, there are certain edge cases that can make the graph not quite match up. The report under general stats is the data that's pulled straight from the "payments and coupons" section of your orders. Because of that, it is the more accurate source than the graphs.
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The category revenue graph in particular will never match the daily revenue graph or the financial stats. This is because it is only looking at the line items in the orders and what categories those items are in. That means that it can't take into account things like returns, tax, shipping, or custom line items in the POS.