When Apple added the ability to see sales and trend reports for the App Store it finally made it possible to see how many people had downloaded your app. However, the sales reports in their raw form contain a lot of data and are difficult to read. For the first few days I was adding up the download numbers manually. However, this is time consuming and can be almost impossible once you have more than one application on the store. That is why I wrote a little utility I call AppStore Clerk to help me out.
Simply place AppStore Clerk in the folder where you keep all of your saved sales reports (you have been saving your sales reports, right?) and it will take care of the rest. You can then take the data and paste it into a spreadsheet if you want to make charts or other fancy things.
AppStore Clerk is freeware, so go ahead and give it a try.
Shameless self-advertisement: If AppStore Clerk saves you a minute or two each day, perhaps you could try out my puzzle game, Lumen.