Gnuplot plots incorrect time for time series plot

2
votes

I'm printing a time series that has the following format

> 1400536853 0.011955 
> 1400537188 0.013695 
> 1400537530 0.010797  
> ....
> 1400621709 0.010688 
> 1400622023 0.007209 
> 1400622338 0.006685 
> 1400622653 0.005539

The first column is a timestamp in unix epoch format and the second is a variable to plot. Notice that the timestamp pf the first line corresponds to "Tue May 20 00:00:53 CEST 2014" and the last line to "Tue May 20 23:50:53 CEST 2014". I veridied this with the following command:

date -d @<timestamp>

I'm using the following script to plot the time series

set xdata time
set timefmt '%s'
set format x '%H'
plot 'series.txt' using 1:2 with lines notitle

But I'm getting a plot with hours in the range 22, 00, 02. ...22 as shown in the next figure:

time series plot

I will appreciate any suggestion on how to fix this. May this has to do with not setting properly the timezone? Many thanks in advance

2
plotgnuplottime-series
I just came across this comment in another stackoverflow's question. Apparently, it's a limitation of gnuplot, which always assumes Universal Time when converting timestamps in seconds from epoch. - pablochacin

2 Answers

6
votes

Gnuplot cannot change the time zone and uses UTC for all time data. This is where your 2h shift comes from:

$ date -u -d @1400536853
Mo 19. Mai 22:00:53 UTC 2014

In your specific case, you can just add the two hours in the using statement:

set xdata time
set timefmt '%s'
set format x '%H'
plot 'series.txt' using ($1 + 2*60*60):2 with lines notitle

Beware, that this explicitely assumes, that you run the script only on computers in the same time zone. I think there is no generic way to convert between time zones within gnuplot, since it doesn't support them.

4
votes

Following Christoph's answer, I figure out how to calculate the offset in a generic way in the script that call gnuplot:

   LOCAL=$(date)
   UTC=$(date -u -d "$LOCAL" +"%Y-%m-%d %H:%M:%S")  #remove timezone reference
   UTCSECONDS=$(date -d "$UTC" +%s)
   LOCALSECONDS=(date -d "$LOCAL" +%s)        
   TIMEZONEGAP=$(($LOCALSECONDS-$UTCSECONDS))

Then in gnuplot I can use this gap as proposed