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To plot a data file which contains date or time, you need to tell
gnuplot that the data are date/time type by the command set
xdata time or set ydata time , then specify the
data file format by set timefmt . Let’s begin with the
following example:
2004-02-09 310
2004-02-10 185
2004-02-11 239
2004-02-12 132
2004-02-13 85
2004-02-14 57
2004-02-15 8
A corresponding timefmt to read this data is “%Y-%m-%d”.
In this case zero’s are padded when month and/or date are less than
10, but gnuplot can read data without this padding (2004-2-9, for
example).
gnuplot> set xdata time
gnuplot> set timefmt "%Y-%m-%d"
gnuplot> plot "sample.dat" using 1:2 with boxes
Reading date/time data with gnuplot is rather flexible. Here are
some examples of the format given for timefmt.
| data |
timefmt |
comment |
| 2004/4/6 |
%Y/%m/%d |
2004/04/06 works well |
| December/96 |
%B/%y |
warning if mis-spelled |
| 2004/Jan |
%Y/%b |
3-letters abbreviation |
| 1970/240 |
%Y/%j |
“%j” is a day of the year (1-365) |
| 02:45:03 |
%H:%M:%S |
“%H”, 24-hour |
| 1076909172 |
%s |
seconts since 1/1/1970 00:00 |
When gnuplot reads in date/time type data as X-axis data, it
generates appropriate X-tics automatically. In the example above,
which are time sequence data for one week, gnuplot writes
“month/day” on the X-axis, plus time of “00:00” which we may not
need. To control the output format, use set format .
This format specification not the same as usual numerical output, but it is a
format for the strftime function of UNIX library.
gnuplot> set xdata time
gnuplot> set timefmt "%Y-%m-%d"
gnuplot> set format x "%b/%a"
gnuplot> plot "sample.dat" using 1:2 with boxes
In addition to the format-characters shown above, you can use “%a”
which does not exist in timefmt . With the “%a” format
gnuplot gives you a weekday name. See manual of strftime function
in detail. Anyway, formats such as “Y, y, m, d, H, M, S” are
probably enough for almost all cases.
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