Adding unwrap to gnuplot

[Jon G. <jo...@th...>]

Hi all!

When plotting phase data in gnuplot, you often encounter "jumps" in a
graph when a value wraps around 2π. A common way of dealing with this is
to apply an unwrap[1] function. What this function does is add or remove
2π from the next value to be plotted (v) as long as the difference
between v and the previous value (w) is outside the range [-π,π].

In pseudocode:

    unwrap(v, w):
      do
        diff = v - w
        v += 2π if diff < -π
        v -= 2π if diff > π
      while abs(diff) > π

It is also possible to implement a weighted unwrap which does the same
as the above, but instead of looking at the previous value w, it looks
at a weighted average of previous values instead. This works very well
for datasets with very low variance.

I'd love to implement this feature in gnuplot myself, but it's not
entirely clear to me how I would go about it. It seems like
standard.{c,h} would be where to start, but from what I can tell, these
functions only receive the current data point, and does not have any
knowledge of previous points. Adding a static variable to track the
previous value seems like an ugly solution, so I was hoping someone here
could point me in the right direction?

Cheers,
Jon

  [1] http://www.mathworks.co.uk/help/matlab/ref/unwrap.html