Some way to differentiate (1st order derivative)?

[Levente <ln...@dr...>]

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.


Levente