How do I extrapolate a line and plot it?

I'm trying to extrapolate a very linear data set using the interp1 function and I'm getting weird results. I can do this in Excel (using the trendline feature with a forward forecast) but I want to add this to an existing Matlab code that I have. Here's a pic of the Excel method (first image ... I was hoping they'd be inline).

However, I get weird results when I do this with Matlab (second image).

Here's the command that I use to grab the last 300 points of my data and use it to linearly extrapolate it forward (with vector Xextend).

Xextend = X(end) Ynew = interp1(X(end-300:end),Y(end-300:end,:),Xextend,'linear','extrap');

Any ideas why it doesn't extend the line like I expected?

Answer 1

[David Goodmanson]

Hi Michael,

Since you are using Xextend = X(end) I don't see how you are extrapolating out to X = 2100. However, if Xextend were equal to 2100, then the problem with interp1 is that it merely takes the slope between the second-to-last point and the last point and uses that to extend out to X=2100. It’s doing exactly what it says, but the history of points between (end-300) and (end-2) is not even considered.

If Xextrap is a set of X values between X(end) and X=2100, what you are looking for is probably more along the lines of

c = polyfit(X(end-300:end), Y(end-300:end),1);   % linear fit
Yextrap = polyval(c,Xextrap);

[Stephen23]

Michael's "Answer" moved here:

It should've been Xextend = X(end):1:2100 but either way I guess I was incorrectly interpreting what interp1 was doing, assuming that it took the entire span instead of simply last 2 points.

I was investigating the polyfit idea when I saw your reply so it looks like that will be the best way. Thanks!