Plotting time series + offset not working - time-series

Question

I want to plot two time series that are in different formats in the
same format, but one of those two doesn't work for me.
So here's the thing:
I have a time series in seconds with a given starting time t0.
So the series looks like this:
Starting time: t0
0 -21.0028
5 -21.0067
10 -21.007
...
17875 -20.9943
I want to plot this time series with the time (e.g. 9:03) instead
of seconds and I want to show another time series where the
data is given in standard time in the same plot.
I can plot the second series just fine.
But when I try to plot the first series, the measured points
start jumping from x=100 back to 0 and then back and forth.
Additionally I can't add the offset t0 to the series.
Here's what I tried (t0 = 32615s) to get at least the first time series plotted:
gnuplot> set xdata time
gnuplot> set timefmt "%S"
gnuplot> set format x "%H:%M"
gnuplot> plot 'measurement.dat' u (timecolumn(1)+32615):2 w l, 'measurement2.dat" u 1:2 w l
Does anyone know how I can plot these time series?
Thanks in advance!

Answer 1

My understanding of your problem is that you have two data sets with different timedata. One in seconds and the other what you call "standard", I assume, e.g. 09:03:00.
And you want to shift the one with seconds by some value.
Internally, dates and times are stored as seconds from January 1st 1970 00:00:00.
Since the first dataset is already in seconds just add your shift-value.
You need to convert ´$Data2´ into seconds via timecolumn(1,"%H:%M:%S").
And the way you display the xtic labels is set via set format x "%H:%M" time. Also check help timecolumn. It also depends if you want to display a time of the day, i.e. 00:00 to 23:59 or if you want to display hours >24h. Then you might want to use "%tH:%tM:%tS"as format. Check help time_specifiers.
Code:
### shifted time data
reset session
# create some test data
set print $Data1
do for [i=1:20000:200] {
print sprintf("%d %.3f",i,sin(i/2000.))
}
set print
set print $Data2
do for [i=30000:50000:200] {
print sprintf("%s %.3f",strftime("%H:%M:%S",i),2*sin(i/2000.-2.5))
}
set print
set format x "%H:%M" time
set key top center
set multiplot layout 2,1
plot $Data1 u 1:2 w lp ti "Data1", \
$Data2 u (timecolumn(1,"%H:%M:%S")):2 w lp ti "Data2"
t0 = 30000
plot $Data1 u ($1+t0):2 w lp ti "shifted Data1", \
$Data2 u (timecolumn(1,"%H:%M:%S")):2 w lp title "Data2"
unset multiplot
### end of code
Result: