Explanations / Notes:
Group and Super Group IDs can be found here.
For ID, a plain number (4) or number prefaced by Gr (Gr4) indicates a Group ID. A number prefaced by S (S4) indicates a Super Group ID.
Size: At Size 1, the idea is that each pixel of bar height will represent 1000 sales. At Size 0.5, they'll be half that tall. This does mean with the default setting a game that sells 4 million will make a 2000+ height image. With big enough groups with a high enough max, you might get an error until you lower this a bit.
Abbreviations are necessary because pulling a full group title from the database would be much too big for the legend, and any sort of automated abbreviation generator would result in much goofiness.
The Sum Through Date is for getting a group as it was at some point in the past. This could allow you to make more even comparisons; perhaps the sales from a new system with a legacy system through its first 9 months or whatever. The sum through date will use the latest data from the last full week on or prior to the date given. Put in 2011-12-31 and the last week of data will be that of December 19-25.
If you use groups that share some games, those games will end up represented multiple times with different colors.