How do I take the derivative of my plot?

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Alejandro
Alejandro 2013-4-19
编辑： Torsten 2023-1-23
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?

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Azzi Abdelmalek
Azzi Abdelmalek 2013-4-19
dy=diff(y)./diff(x)
plot(x(2:end),dy)
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Ali
Ali 2023-1-23
does that work when the derivative at some data points has 2 values (before and after the data point)?
I mean the derivative of the plot is not continuous in all points.
Torsten
Torsten 2023-1-23
编辑：Torsten 2023-1-23
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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Shaun VanWeelden
Shaun VanWeelden 2013-4-19
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
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John D'Errico
John D'Errico 2016-7-31
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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