Added Color to Scatter Chart

Hello,

I'm trying to color a scatter chart similar to the one in the attached spreadsheet (red, yellow, blue, green)
The colors should follow the functions y=x and y=-x. For my chart, i used a static image as the background. However, if i need to adjust my axis and center point to be skewed then the static picture obviously wont adjust.

Is there another way to do this?

Thank You