Hahaha I hate how you can't undo things too! That's why it takes a while to get it. Actually I ended up drawing this out on graph paper to so I wouldn't have to guess and redo it so many times.
But the good news is that if you do create something awesome, you can save it in the memory of the calculator so you'll never lose it!
[2nd]+[DRAW] Draw Menu
[<] Sto Menu
[ENTER] 1:StorePic
and then just save it to some number and when you want to see it again, go to the same place except instead of doing a 1:StorePic, do a 2:RecallPic and type in the number and bada-bing bada-boom, up comes your picture!