Qplot for Gnuplot

Gnuplot

Gnuplot is a very full-featured graphing/plotting program, with the added attractions of being free and open-source.

Find out about gnuplot at www.gnuplot.info.
Download it from ftp.gnuplot.org or (perhaps better) sourceforge.
Obtain a full manual in (gzipped) pdf format here.

One limitation of gnuplot from an economist's point of view is that it is not set up to handle quarterly time-series data with ease. You can plot such data OK, but it's difficult to get the time-axis tic labels to come out right (e.g. "1960.1", "1995.4"). Gnuplot actually has a sophisticated and flexible mechanism for handling time on the x-axis, but it is based on the unix time "struct", which does not recognize the quarter-year as a primitive unit of time, so you have to go through some contortions to get the labels right.

The qplot package

The little package here is designed to make the plotting of quarterly data easy. It takes the form of two files:

a perl script, gp_quarters.pl that acts as a "pre-processor" for a quarterly time-series data file, putting it into a form suitable for use with gnuplot, and also writes out a set of gnuplot commands for creating the time-axis tics and labels. The modified dataset is written to a temporary file.
a gnuplot command file qplot.gp that interfaces with the perl script. This file can be called from the gnuplot command line, or from within an existing gnuplot command (batch) file.

How it works

Here is the modus operandi:

First ensure that your data file meets the required specification: the first column should contain quarterly dates in the standard economist's format, e.g. 1960.1, 1995.3, 47.1 (years can be given in 4 or 2 digits; quarters are given as 1 digit following a "decimal" point). The remaining columns can contain any number of data series. Any "comment" (non-data) lines should have a "#" in the first column. A little example is given in the file q2.dat.
Ensure that gp_quarters.pl is in your path, and that qplot.gp is in the current working directory.
In gnuplot, issue a command of the form:
```
call "qplot.gp" "<datafile>" "<plot parameters>" {<maxtics>}
```
For example
```
call "qplot.gp" "econ.dat" "using 1:2 with lines" 12
```
Note that the first 3 fields after call must be quoted. Supply the name of your original datafile: the perl script will write out a temporary modified version (qdata.tmp) and use this for the plot. Then supply the parameters to the plot command that you wish to use, as usual but quoted (see the gnuplot manual for details). Finally (and optionally) supply an integer representing the maximum number of date-labeled tic marks to place on the x-axis. This defaults to 12.
If you get an error message saying that the files tictmp.gp and qdata.tmp are missing that means that the perl script has not done its job, probably because it can't find perl. Please open gp_quarters.pl in a text editor and fix it for your system.

Requirements

The qplot script uses the "call" command in gnuplot. This requires gnuplot version 3.7 or higher.
The main work is done by perl, so you need to have perl installed.
If you're using a non-un*x operating system, you'll probably have to edit qplot.gp, changing the "rm" to "del" to get rid of the temporary files that qplot generates.

Downloading qplot

You can simply grab the files mentioned (and linked) above. Or you can grab a zipfile containing everything, including a copy of this page and some sample datafiles: qplot.zip. Qplot is governed by the GNU Public License.

Sample plots

These simple plots are designed to illustrate the date handling capacity of qplot.

Sample 1:

set size .8,.8
set term png color
set output "sample1.png"
set title "Gross domestic product (AR, bil. 1987 $) 1948.1 - 1993.3"
set nokey
call "qplot.gp" "q3.dat" "using 1:2 w l"

Sample 2:

set size .8,.8
set term gif
set rmargin 4
set output "sample2.gif"
set title "Gross domestic product (AR, bil. 1987 $) 1948.1 - 1950.4"
set nokey
call "qplot.gp" "q2.dat" "using 1:2 w l"

Sample 3:

set size .8,.8
set term png color
set rmargin 4
set output "sample3.png"
set title "Gross domestic product (AR, bil. 1987 $) 1948.1 - 1950.4"
set nokey
call "qplot.gp" "q2.dat" "using 1:2 w l" 6

Utility

Do you have a datafile containing quarterly time series data, but without the required date markers in the first column? Here is a little perl filter that adds dates: add_quarters.pl. The useage is, for example:

add_quarters.pl 1956.1 < econ.dat

That is, supply a starting date in the form YYYY.Q (or YY.Q) and supply the name of an existing datafile on standard input: the datafile with dates added will appear on standard output.

Allin Cottrell (cottrell@ricardo.ecn.wfu.edu) May, 1999