MATLAB: Get intermediate data from plot

datafiguregetting data from plotgraphintermediate datainterpolationobtaining dataplotplots

Hello;

I'm having problems with obtaining intermediate data from a Figure (plot). I have two vectors (Recorregut & Velocitat) which I have represented in the following figure as if I was working with Excel.

The thing is I have "x data" such as 24.54, 35.62, etc. and I would like to know the intermediate "y-data" value for every value of a vector x (x values starting at 0 and ending at 1700 with intervals of 2 for instance, that is to say, pieces of data which I do not know but I see in the figure) that Matlab plots in spite of the fact that I know that the spline that Matlab shows is not "real" data.

Is there a way to obtain all this data in a vector or in a matrix? If you attach some code I would be eternally grateful.

Thank you so much.

Yours

Catriona

Best Answer

Use the interp1 function. Assuming ‘Recorregut’ is your independent variable and ‘Velocitat’ is your independent variable, you can get any intermediate variable.

For example:

Recorregut_i = 0 : 2 : 1700;
Velocitat_i = interp1(Recorregut, Velocitat, Recorregut_i, 'linear');

The ‘Velocitat_i’ vector will have values for every value of ‘Recorregut_i’.

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MATLAB: How to fix error about interp1

One way (without seeing your data, so this is a guess on my part):

y85 = find(y <= 85, 1, 'last');
yi = interp1(y(y85-1:y85+1),x(y85-1:y85+1),xi, 'linear','extrap');

That is how I would do it. This assumes there is only 1 where ‘y’ is approximately 85.

Note — This is obviously UNTESTED CODE. It should work, if there is one ‘y’ near 85.

EDIT — Added the 'extrap' option to make my approach more robust.

MATLAB: Does the INTERP1 function in MATLAB handle complex data with the “cubic” method

This enhancement has been incorporated in Release 2006a (R2006a). For previous product releases, read below for any possible workarounds:

The cubic interpolation algorithm is not intended for complex data and hence does not behave well under certain circumstances when the data is complex. As a workaround, use the spline interpolation instead.

Y_spline = interp1(X,Y,Xi,'spline');
plot(X,abs(Y),'b',Xi,abs(Y_spline),'r')

Alternatively, you can split Y into real and imaginary parts, and then interpolate.

Yr = real(Y);
Yi = imag(Y);
Y_cubic = interp1(X,Yr,Xi,'cubic')+i*interp1(X,Yi,Xi,'cubic');
plot(X,abs(Y),'b',Xi,abs(Y_cubic),'r')

Best Answer

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MATLAB: How to fix error about interp1

MATLAB: Does the INTERP1 function in MATLAB handle complex data with the “cubic” method

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