How to create Custom Median line on Scatter Chart

Hi,

I am creating scatter chart based on some variance calcualation. Everything is good just need to draw custom line in that chart where I need some help.

For example I have this scatter chart:

As you can see all values are above 0.7 but median line is showing at 0.75. 

I would like to create custom median line on 0.7 so i can show all above 0.7 as green dots and below 0.7 as red dots.

Everything is working except Custom MEdian line. Is there a way I can create Custom median line?

Thanks