I have a tricky question regarding to what i'm trying to do.
I have a plot with two lines (the mean of two conditions) on it. I want to add on the same plot a square reflecting the t-values (and colored according to these values in a gradient way). How could i add this square?

Well since i don't know if i'm clear, here is a figure of what i try to achieve.

Thank you for any help!

![enter image description here](https://i.stack.imgur.com/vDgJy.png)