I actually use TestComplete in an application that contains a grid with a line running through that grid and points on the grid (So it might be similar to a line graph).
How I do this is I have a base setup for this grid. I can a function to set it back to that default. Then I take a picture of it like:
set pic1 = gridControl.picture
Then I make adjustments to the line on the grid. Then I do:
set pic2 = gridControl.picture
Once I have both pictures I use the Compare method to compare both images. In my case, I allow for a certain amount of error by using the Threshold option (I usually set it to 50 or so for 50 pixels can be different).
The compare should return a True or False. If it's False then you can use log.picture(Pic1.Difference(pic2)) like that to log a picture of the differences for review.
Also, if you have something pre-determined, you can snap an image of that and add it to the Regions under Stores and then compare directly against this item in your stores.