Adding Two Extra Lines to a Scatter Plot

I have a scatter plot exactly like the one in the picture I have attached, except mine does not have the two dotted lines. My plot has the best fit line in the middle and I am trying to figure out how to add those two extra dotted lines. I do not know what to even call them and that is why I have attached the picture. My goal is to add those two dotted lines into my plot.

[Jim Riggs]

The question is unclear.

Do you want to know how to add a line to an existing scatter plot, or do you want to know how to calculate the ordinates of the two other lines?

Answer 1

[Star Strider]

I am not certain what the dashed lines are. If they are confidence bounds on the centre regression line, the File Exchange contribution polypredci (link) function can calculate them.