GNUPlot is an open-source versitile data plotting and mathematical charting application. Timeline plot is the most common type of a plot you would generate. It also seems that it’s one of the more capricious features of GNUPlot. Here are a few simple examples to get you over some problems that otherwise may not be obvious.

The most common date/time format used by most databases and applications looks something like this: 2006-04-13 11:42:12; or in general form: YYYY-MM-DD hh:mm:ss. Below is a generic data sample where the first column is date/time, while the second, third and fourth columns represent three different types of data. Each value in clumns 2, 3, and 4 is, therefore, a function of time. Let’s also suppose that this data file is /tmp/data1.dat.

Create a new text file and call it, say, /tmp/plot1.gnu.

And now to plot it:

If you paid attention, you should now have a /tmp/plot1.png file that shows a plot of your three data types as a function of time.

Let’s review the options in the /tmp/plot1.gnu file line by line.

set title ‘Title of Your Plot’
This is simply the title of your plot. It will appear at the top of the resulting image.

set xdata time
You need to tell GNUPlot that you desire a timeline type of a plot.

set key box
GNUPlot will create a little box with the plot key. It’s useful for knowing which line represents what data.

set key bottom right
And, of course, you want to tell GNUPlot to place the key in an appropriate location so it doesn’t clash with the rest of the plot.

set size 1.5,1.5
The desired size of the resulting plot image in mysterious units.

set xlabel ‘Time’
X-Axis label

set ylabel ‘Data’ font ‘Arial,12′
Y-Axis label and font type/size. You can also specify font type and size for the X-axis.

set autoscale
Unless you want to plot only a specific chunk of data (say, from last Monday until today), you should tell GNUPlot to automatically scale the plot to include all available data.

set timefmt “%Y-%m-%d %H:%M:%S”
Now, this is important, so pay attention. Here you tell GNUPlot about the format of your date/time field. Because date and time are usually separated with a space, you need to include the whole thing in quotes.

set term png color
Here you express your desire for a color plot, unless you are into black-and-white.

set output ‘/tmp/plot1.png’
This is where GNUPlot will store the resulting image.

plot \
‘/tmp/data1.dat’ using 1:3 title ‘Data 1’ with linespoints, \
‘/tmp/data1.dat’ using 1:4 title ‘Data 1’ with linespoints, \
‘/tmp/data1.dat’ using 1:5 title ‘Data 1’ with linespoints

OK, so here is the most important part. The “using 1:something” part tells GNUPlot that your X-Axis value (date and time in this case) is in the first column and your data is in clumns 3, 4, and 5. And that you would like to plot all three data types as separate lines that would also show data points (the linespoints option).

But shouldn’t it be “using 1:2″, “using 1:3″, and “using 1:4″ instead, since your data contains only four columns: date/time, data1, data2, and data3? No it shouldn’t. Keep in mind that your date/time column is in fact two columns: date and time are separated by a space. So there you go.