Thanks for uploading! Helps to see that.
We just uploaded a file that includes these two graphs. Thought you might be interested in also trying out an oval, in addition to a rectangle. (In DataGraph, go to File > On-line examples to download. It will be at the top of the list.)
The highlighted area is drawn with a Region command.
The file is annotated so hopefully you can follow what we did.
Please take a look and let us know if this helps and any other questions.