# How to find the slope of phase data (between two points) which is uneven

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Sriram Guddati on 12 Oct 2018
Answered: Greg Dionne on 26 Oct 2018
I have a phase vs frequency plot. I need to find the slope between point 'a' and point 'b' in an automated way instead of looking at the points 'a' and 'b' and calculating the slope. Can anyone help me with this?

Image Analyst on 12 Oct 2018
You ask ONLY about the slope, not about finding a and b, so I assume you've found "a" and "b" already. So, assuming your "a" is index1 and "b" is index2, you can do this
coefficients = polyfit(x(index1):x(index2), signal(index1:index2), 1);
slope = coefficients(1);
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Image Analyst on 12 Oct 2018
OK, that wasn't stated in the original question. So you can use
[peakValues, indexOfPeaks] = findpeaks(signal);
[valleyValues, indexOfValleys] = findpeaks(-signal);
valleyValues = -valleyValues; % Flip again to get right side up.
Of course you will find lots of peaks and lots of valleys so you'll have to decide which to concentrate on.
Sriram Guddati on 12 Oct 2018
Thank you, I will try this.

jonas on 12 Oct 2018
Edited: jonas on 12 Oct 2018
I would suggest using either findpeaks to determine points a and b and then calculate the average slope or alternatively using findchangepts. If you want code, then I suggest you upload some data.
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jonas on 12 Oct 2018
Yes of course. The index is the second output of findpeaks. Note that you need to run the function two times, one for peaks and one for valleys. For the latter, you just turn the plot upside down, i.e.
findpeaks(-y)
Sriram Guddati on 12 Oct 2018
Thanks, I will try this method

Greg Dionne on 26 Oct 2018
You could try a combination of unwrap (to remove the jumps of your array) and sgolay to do the differentiation. See the second example on how to perform differentiation.