**Very Basic GNUPLOT**
**Chuck Gartland**
**September 2001**
##
Introduction

GNUPLOT is an easy-to-use interactive plotting package installed on the
Math/CS computer network. It is entered with the command
`% gnuplot`

```%`'' is the UNIX command prompt. Exit with either of the commands
`gnuplot>` `exit` or
`quit`

```gnuplot>`'' is the GNUPLOT command prompt. GNUPLOT is a public-domain
package, available via anonymous `ftp` from
`ftp.dartmouth.edu`
in the subdirectory `/pub/gnuplot`. There is a brief man page
`% man gnuplot`

and an extensive on-line help facility available inside the program:
`gnuplot>` `?` or
`help` or `help
`*topic*

First time users should peruse the man page and should read
`gnuplot>` `help plotting`

GNUPLOT draws pictures of two- and three-dimensional functions and data
in windows on your workstation screen. It can also write to disk PostScript-language
plot files, which can be printed on the laser printers. This very brief
introduction discusses (through a few sample commands) how to use GNUPLOT
to plot functions and to do simple ``connect the dots'' plotting of data
from a disk file. It also discusses how to print out hard copy of output
on a laser printer.
##
Plotting Functions

`gnuplot>` `plot sin(x)`

will plot the function *sin(x)*, on the default domain [-10,10].
To specify a different domain, one can use (for example)
`gnuplot>` `plot [x=0:2*pi] sin(x)`

##
Connecting the Dots

`gnuplot>` `plot "plot.dat" with lines`

will draw a simple 2-D plot, connecting the dots with straight lines, of
the data in the local file ``plot.dat''. This file must have been generated
previously (usually as the output from some other user program) and in
the current working directory (or prescribed with its full path name).
The contents of this ascii file should be two numbers (separated by spaces)
per line--any number of lines--representing the *(x,y)* coordinates
of the data points to be plotted.
`gnuplot>` `plot "plot1.dat" with lines, "plot2.dat"
with points`

will plot both ``plot1.dat'' and ``plot2.dat'' on the same plot, using
lines for the first data set and points for the second. The same effect
can be accomplished with the following two commands in succession:
`gnuplot>` `plot "plot1.dat" with lines`

`gnuplot>` `replot "plot2.dat" with points`

For more information on these basic and important plotting commands:
`gnuplot>` `help plot`
and `help replot`

##
Hard Copy Output

Once you have your plots looking as you wish in your viewing window, the
following sequence of commands can be used to generate a PostScript-language
disk file (here called ``plot.ps'') for laser printing hard-copy output:
`gnuplot> set terminal postscript`

`gnuplot> set output "plot.ps"`

`gnuplot> replot or
`*original plotting command(s)*

The generated disk file can be laser printed (on your default printer)
with the command (outside of GNUPLOT, at the UNIX level)
`% lp plot.ps` (HP-UX)
or `% lpr plot.ps` (Linux)

or on the selected printer `graphlab` (for example), via
`% lp -dgraphlab plot.ps` or
`% lpr -Pgraphlab plot.ps`

For further GNUPLOT ``window plotting,'' you must reset these variables
to their defaults by using the commands
`gnuplot> set terminal x11`

`gnuplot> set output "STDOUT" or
set output`

For more information on this other basic command, and to see what other
kinds of things can be ``set'':
`gnuplot>` `help set`

Chuck Gartland /
*gartland@math.kent.edu*
September, 2001