How to add offset to time-based data when plotting in gnuplot

Question

I want to add an offset to the time-based data from a file which I want to plot using gnuplot. I can plot the data just fine, but when I try to add a time-based offset the graph is empty.

The purpose is to plot multiple bars next to each other, like described here, but with time-based data: Two plots "with boxes" next to each other with gnuplot

Data file:

00:00 7719
01:00 20957
02:00 15989
03:00 9711
04:00 1782
05:00 871
06:00 4820
07:00 860
08:00 873
09:00 848
10:00 879
11:00 726
12:00 944
13:00 924
14:00 996
15:00 806
16:00 848
17:00 967
18:00 2277
19:00 2668
20:00 32183
21:00 14414
22:00 20426
23:00 16140

I am trying to plot the data using this code:

set xdata time
set timefmt "%H:%S"
set format x "%H"
set style fill solid 0.6 border -1
set boxwidth 0.3 relative
plot ["00:00":"23:30"] 'data.dat' using ($1-0.3):2 with boxes, \
  'data.dat' using ($1+0.3):2 with boxes

This is only a test - the real data file has additional data columns, and I am trying to place the boxes next to each other using offset, however I'm having no luck with time-based data and offsets.

Without offset the code is fine:

set xdata time
set timefmt "%H:%S"
set format x "%H"
set style fill solid 0.6 border -1
set boxwidth 0.3 relative
plot ["00:00":"23:30"] 'data.dat' using 1:2 with boxes

Answer 1

For computations with timedata you must use the timecolumn() function, which parses a time string from a column according to the set timefmt settings. The result is a time stamp in seconds. So, for the offset you must use the according time in seconds:

set xdata time
set timefmt "%H:%S"
set format x "%H"
set style fill solid 0.6 border -1
set boxwidth 0.3 relative
set xrange["00:00":"23:30"]
set style data boxes
plot 'data.dat' using 1:2, \
     '' using (timecolumn(1)+60*20):($2*0.5), \
     '' using (timecolumn(1)+60*40):($2*0.7)

This gives:

As another variant you can use the histogram style, and format the xtic labels using strftime and timecolumn:

set timefmt "%H:%S"
set style fill solid 0.6 border -1
set style data histogram
set style histogram clustered gap 1
plot 'data.dat' using 2:xtic(strftime('%H', timecolumn(1))), \
     '' using ($2*0.5), \
     '' using ($2*0.7)

That gives:

You get no separation between two data blocks with set style histogram clustered gap 0.

In order to control the tic labelling, you may use something like

plot 'data.dat' using 2:xtic((int($0) % 2 == 0) ? strftime('%H', timecolumn(1)) : '')

which prints only the label of every second entry in the data file ($0 refers to the row number).