MATLAB: How to get values out of a scatter plot that are in between the data points

MATLABplotspline demo

Ok, so I have a dataset with a column of x values and a column of y values. I have plotted them as a scatter plot here.

What I want to do is extract x values for a given y value. E.g. what are the x values when y = 17? As you can see, a given y value might not actually have a "plotted" x value.

Best Answer

You could try spline() or interp1(). See attached demo.

Red points are the original "knots" and the blue points are the interpolated points.

You can maybe reverse the x and y and plug the y into spline to get the x.

Or else make a few million interpolated points and use ismembertol(), though there may be a more direct and accurate way such as finding the two points on either side of hte y value and using bilinear interpolation.

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Use interp1,

desiredY = interp1(currentX,currentY,desiredX)

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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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