glitch in cosine

[<pl...@pi...>]

Hi,

I have a curious glitch in plotting the sum of three cosines.

set samples 1700

            twopi=2*pi
           c1(x)=a1*cos(twopi*(x-yz+ph1)/p1)
           c2(x)=a2*cos(twopi*(x-yz+ph2)/p2)
           c3(x)=a3*cos(twopi*(x-yz+ph3)/p3)
           f3(x)=c1(x)+c2(x)+c0+c3(x)

  a1=a2=a3=1; c0=3.; yz=1958; p1=1.0;p2=0.5;p3=.25; ph1=ph2=ph3=.1
  a3=1;ph3=0.1;p3=0.25

fit f3(x) datafile   u (($1)/365.25+1945):2 via 
a1,a2,c0,ph1,ph2,p2,p3,a3,ph3

  plot  datafile u (($1)/365.25+1945):(f3($1)) w l


data either side of that point seem fine and if I use it directly with 
plot "-" I don't get the glitch.


Closing gnuplot session and repeating the plot from file gives same 
result seen in attached png flie.

For the benefit of the list that don't see the file, it looks like a 
cosine ranging -0.07 and + 0.014 pk to pk with some small ripple with 
sporadic spikes ranging from 0.025 to 0.075 , glitches are just single 
points.

Irrespective of the actual data fed in , I fail to see how the sum of 
cosines can have such glitches.


If anyone can explain how that could happen I'd like to know, otherwise 
it looks like a bug.

Thanks, Peter.





   plot  "-" u (($1)/365.25+1945):(f3($1)) w l

22629  381.81
22636  381.88
22643  381.65
22650  382.62
22657  382.25
22664  381.37
22678  383.09
22685  383.59
22692  384.19
22699  383.76
22706  383.71
22713  383.64
22720  384.28
e




Final set of parameters            Asymptotic Standard Error
=======================            ==========================

a1              = 0.0486237        +/- 0.001654     (3.402%)
a2              = 0.0265777        +/- 0.001788     (6.728%)
c0              = 0.00416031       +/- 0.001169     (28.1%)
ph1             = -0.0557476       +/- 0.005409     (9.703%)
ph2             = 0.163801         +/- 0.009924     (6.059%)
p2              = 0.49982          +/- 0.0001585    (0.03171%)
p3              = 0.258274         +/- 0.0005954    (0.2305%)
a3              = -0.00192983      +/- 0.00202      (104.7%)
ph3             = -0.923156        +/- 0.06458      (6.996%)

  G N U P L O T
         Version 4.7 patchlevel 0    last modified 2012-10-16
         Build System: Linux i686