Getting data values from a plot

Hi Guys,

I want to get (x,y) values from a plot. Say my plot is made of the following points (1,1) (2,3) (4,4) and (5,9). Is there anyway to get the value of y on this plot when x=2.5.

Thanks NS

Answer 1

[Nathan Greco]

User interpolation

   x = [1 2 4 5];
   y = [1 3 4 9];
   xi = x(1):.01:x(end); %values between x
   yi = interp1(x,y,xi);
   findyatx = 2.5;
   yfound = yi(abs(xi-findyatx)<eps);

[NS]

Thanks Nathan.

Is this interpolation valid only for integer X and Y values. What about the case of non integer values of x. I am getting the following error.

"??? Error using ==> interp1 at 261

The values of X should be distinct."

[Sean de Wolski]

Distinct means no duplicates. Take the mean of any duplicate values. (unique and accumarray will be your friends)

[NS]

Works really well. Thanks Nathan

Answer 2

[Sean de Wolski]

You mean fit a line to it and solve?

Sure, use polyfit and polyval.

doc polyfit
doc polyval

[NS]

Thanks. It works ok. Is there any way to get the best degree for the polynomial automatically. I need to do this for a plot of plots and the curve fitting maybe different for each.

[Sean de Wolski]

"Best" is a word you could write a PhD thesis on.

[NS]

Hehehehhe....Ok..