How do I take the derivative of my plot?

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Alejandro
Alejandro on 19 Apr 2013
Edited: Torsten on 23 Jan 2023 at 21:36
I have my temperature in the y axis, and my distance in the x. I have them all plotted out and I have the data too, how can I take the derivative of the plot?

Accepted Answer

Azzi Abdelmalek
Azzi Abdelmalek on 19 Apr 2013
dy=diff(y)./diff(x)
plot(x(2:end),dy)
  8 Comments
Torsten
Torsten on 23 Jan 2023 at 21:31
Edited: Torsten on 23 Jan 2023 at 21:36
If you only have discrete values for x and y, there is no method to tell you whether there is a discontinuity in the derivative of y in a point x_i.
You can compare (y(i+1)-y(i))/(x(i+1)-x(i)) with (y(i)-y(i-1))/(x(i)-x(i-1)), but even if both differ significantly, it's not possible to decide whether there is a discontinuity in the derivative at x=x(i).

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More Answers (1)

Shaun VanWeelden
Shaun VanWeelden on 19 Apr 2013
Call polyfit to generate your polynomial (if you don't already have a polynomial)
Call polyder to get derivative of your fitted line
Call polyval with your original X values to get the Y values of your derivative and plot that with hold on so it doesn't erase your original plot
  1 Comment
John D'Errico
John D'Errico on 31 Jul 2016
Note that polyfit (any polynomial fit) will often be a terribly poor choice here, since many curves are not well fit by a polynomial model. For example, consider points that lie on the perimeter of a circle, or the function sqrt(x), near x==0. Or the function sin(x), over multiple periods. Or a classic problem function for polynomial fits, 1/(1+x^2).

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