#2884 smooth unwrap starts in the negative if initial value is > pi

[Anonymous]

The smoothing should probably subtract or add 2pi if the first value is outside of [0:2pi].

Current behaviour is apparently that the previous point, used to evaluate if an unwrapping is in order, gets initialised to zero (interpol.c:887 : lasty = 0), maybe it's enough to initialise it to pi. Or just skip the smoothing operation for the first datapoint.

Example:

set xr [0:30]
set table $dat                                                       
plot '+' us 1:(fmod(4+(-sin(x)-x),(2*pi))) with table  
unset table                                                          
plot $dat smooth unwrap

While on the subject: could smooth unwrap be extended to have a "period" specifier like kdensity? Default to 2pi, but could be 246060 seconds, or whatever? That'd also solve the issue here https://sourceforge.net/p/gnuplot/patches/799/ without the user being forced to use the global "set angle degrees".
Whenever the value changes by period/2, period gets added or subtracted.

[Karl Ratzsch]

The first datapoint should just be kept as-is, because there is no knowing in advance what the data range actually is, e.g. +-pi or 0 to 2pi. And if somebody has data that goes from 5pi to 7pi, they probably have some extra knowledge the plot should preserve. ;)

Attached the two-liner fix.

I was slightly confused about the while loop below the change, I think it can be removed and always run exactly once. If there is a jump larger than 3pi in some data, then smooth unwrap is obviously not the right treatment.

Test:

set xr [0:10]
set samp 400
mod(a,b) = a - b * floor(a / b)
testf(x) = mod(1.9*pi*cos(x),2*pi)-pi
plot testf(x),  '+' us 1:(testf(x)) smooth unwrap

The test shows another bug: if no yrange is set, the unwrapped plot leaks out of the graph borders.