# How do I find the (x,y) coordinates of the peaks and valleys of a graph?

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Dave Phillips
on 1 Nov 2015

Answered: Sergio Yanez-Pagans
on 28 Mar 2021

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Image Analyst
on 1 Nov 2015

The x values are the second return argument of findpeaks(). It's the index number. Your formula does not have an x by name so you have to go with the index number. If you have a second array for t, then to get the t values you'd do

[peakValues, indexes] = findpeaks(y);

tValues = t(indexes);

To get valleys, you invert the signal, so that now what used to be valleys are now peaks, and use findpeaks() again

invertedY = max(y) - y;

[peakValues, indexes] = findpeaks(invertedY);

tValues = t(indexes);

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Markus Wahl
on 24 Nov 2018

### More Answers (1)

Sergio Yanez-Pagans
on 28 Mar 2021

You can use my MATLAB file exchange function, it's really easy to implement and use:

Hope you find this useful!

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