Other lines parallel to a Best fit line on a scatter graph?

You mean like this:

If that is what you are asking, you need to put the min and max X values in the table, without any Y data, and include those rows in the chart.

When I first read your question I was totally confused what you were asking. Then it sounded kind of like you were asking to extend the linear fit line. Or maybe you actually want to make parallel lines. If you want parallel lines, you will have to create the linear fit line and all your parallel lines in your table instead of using the trendline feature of charts.The SLOPE function will give you the slope of your X-Y data. The INTERCEPT function gives you the intercept. Parallel lines simply have different intercepts with the same slope. Plot those extra columns of data, sharing the X axis data.

Is this what you want>?

Do you have an easy way to do that automatically? I can do it with a complex brute force bunch of formulas but it seems like there should be an easier way than what I have done. I am too ashamed to post it, even though it does work.

Simplified a bit so I am not so ashamed

I2=SLOPE(C2:C9,B2:B9)

I3=INTERCEPT(C2:C9,B2:B9)

Y Linear (column D) =$I$3+B2×$I$2

Y - YLinear (column G) is what it says

Max Line (column E) = $D2+MAX(G)

Min Line (column F) = =$D2+MIN(G)

You have to format the "trend" lines with connecting lines and no data symbol

I drew lines. I think your way is fine. all you need to know is the slope and the

difference between the max and the mean at the same X and

the difference between the mean and the min