Tangent to the plot

[atlas3000]

I have a list of 2-D coordinates
I made a curve by this coordinates
Now i need to make a tangent from a point of the beginning and calculate coordinates of contact my curve and tangent line
I add simple pic, my tangent should look like red line
Does it possible to make it in Excel without a function?
Plz help
1w.jpg

[MrShorty]

What do you mean by "without a function"? Most any approach in Excel to your question that I can think of would involve some kind of function.

It would certainly be possible to determine the endpoints of the tangent line outside of Excel (using a calculator or something), then enter those endpoints into Excel and plot them as a second series to get your tangent line. Is that what you want to do?

[shg]

You could roughly calculate the slope and intercept at a given point using the values of adjacent points. Or, if you have a smooth function (yours looks like a sigmoid), compute the slope & intercept by differentiation. After you do that, though, you'll need to calculate two {x,y} points to plot a series.