Fitting line to a curve

I have a plot of attenuation V frequency and I am wondering if there is a way I can fit a straight line the follows the upper trajectory of the graph, so that you would see the line increase and then level off. I have tried a few methods but getting lines through the middle etc. Many Thanks

[Matt J]

Please explain "upper trajectory". Do you mean the subset of points formed by the oscillation peaks?

Also, I recommend attaching your data in a .mat file so that we can demonstrate solutions.

[David Harra]

Thanks I will keep this in mind for future. How do I include my data as a .mat file?

Dave :)

[Matt J]

The toolbar has a a file attachment option.

Answer 1

[John D'Errico]

Easy.

1. Use findpeaks, to locate the sequence of points that represent the successive maxima of the curve.
2. Use an interpolating spline to then interpolate those points. A tool like interp1 will be simplest. Specify the 'pchip' option.