Re: Some way to differentiate (1st order derivative)?

[Juergen W. <wie...@fr...>]

> AFAIK calculating or approximating the 1st order derivative of a
> function or x-y dataset was not (easily?) possible a few years ago from
> within gnuplot, but maybe this has changed since then -- I don't follow
> closely gnuplot's development anymore.
>
> My problem is the following: from a dataset, I first plot the x-y
> values, then either fit a function to them or make a smoothing with
> acsplines. So far, no problem. But afterwards, I need the 1st order
> derivative of the fitted or smoothed curve. This would be easy if
> gnuplot had a way to access data not only from the _actual_ row but also
> from the following one in order to calculate Delta y / Delta x = y(row
> +1)-y(row)/(x(row+1)-x(row)). This was not possible earlier. Has the
> situation changed since then? If not, is it still possible to perform
> differentiation from within gnuplot (even if this means using some awk
> script or similar), as I would like to automate the process and gnuplot
> is easily driven from a shell script or batch file.

Well, AFAICS it has become possible in 4.3, though not very easily
accessible.

There is a new assignment operator (and a comma operator), so you
can plot something like:

old_x = NaN
plot 'test.dat' using 1:(dx=$1-old_x,old_x=$1,dx)

This only works with data file plotting.  You can use the '+' pseudo
file for fitted data (though I would recommend the analytic
derivative in this case).  For smoothed curves, plot the results
into a table file (help set table) and use this file as given
above.

-- Completely untested --

Juergen