I've got a scattergraph and developed a line of best fit. (For info, it's a
4 order polynomial.)

What I'd like to do is show additional lines on the chart +/- 20% around the
calculated lobf - basically so I can see what points are outside that range.

Is there a way I can do it. Ideally a generic solution would be the best as
the next lobf could as easily be exponential.

TIA

Rob