der.m v1.0 (Nov, 2009)

Estimates first derivative of an array by using POLYFIT locally on each point ignoring NaNs!

DER Estimates the derivative of an array X with respect to its argument T: dX/dT by evaluating the slope of a polynomial fit of elements "close" to each point. This "closeness" is controlled by an optional input called window width, in turn this controls the smoothness.

Syntax:
>> dXdT = der(X,DT);
where X is a time series and DT its sampling interval.
>> dXdT = der(X,T);
if T is not uniform.

Other optional inputs are:
- the window width (W)
- the polynomial fit degree (DEG)
- the padding (PAD) % to improve derivatives at edges

Besides, the error of the dXdT estimate may be obtained.

Enjoy it!
Bugs reports and comments will be very appreciated!