Drawing a graph for REG

[寺岡佑記]

Hello Roger and everyone

I am a hobbyist studying and DIY REG, have no scientific nor mathematical background.

This is my first post in this community and I a question in regard to drawing a graph for REG.

Currently I am trying to process input generated from REG into a graph like the image below. But I cannot figure out how to draw this blue line indicates P-value...

I would like to know how I can derive this blue line or perhaps could you please share me the formula? 

Thank you very much!

Yuki Teraoka