Re: Adding unwrap to gnuplot

[Ethan A M. <sf...@us...>]

On Thursday, 30 January, 2014 14:00:18 Jon Gjengset wrote:
> 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. 

Have a look at the running average demo:

  http://gnuplot.sourceforge.net/demo_4.6/running_avg.html

Substituting an unwrap function for the avg5() function should do what you want.
It would not require any changes to the executable code.

One problem I see is that the resulting plot would not stable when
zoomed.  Each change in xrange would potentially shift the whole curve
on y by some multiple of 2π.

> 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?

I don't think that adding a new function with a single argument is what
you want.   It seems to me this is more like a trivial smoothing operation,
and the easiest fit into gnuplot's existing organization would be

   plot $DATA using 1:2 smooth unwrap with lines

If you want to work on implementing this in the code, start by looking in
plot2d.c for switch statements that handle SMOOTH_NONE or its
alternatives.

	Ethan

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