Howdy! I set out to simplify the graph in your activity but ended up re-discovering why it was so complicated in the first place 
Anyways, here is a modified copy with the negative algebra blocks:
One known quirk is that if a trip balances out to 0 (e.g. one x^2 block and one -x^2 block), the display will read 0x^2 + 0x + 0. This is because of what I fed into formatLatex to set up the display, and it could be changed by adding in a when...otherwise... statement to the point labels if it bothers you. I hope this helps!