Gnuplot

Problem: Where can I find the latest compiled version of Gnuplot for my platform?

Problem: Printing is kind of annoying in Gnuplot. First you have to set the terminal type and redirect the output, then plot the graph, and finally reset the terminal and output back to their defaults--a minimum of five commands. Is there a faster way of doing this?

Problem: Is there a "frequently asked question" (FAQ) page for Gnuplot?

Problem: Is there a way to print multiple plots on the same graph?

Solution: That depends. Version 3.6 supports the command set multiplot which does exactly that and apparently there are some extensions to Gnuplot 3.5 which can also do it but if you want to print to an encapsulated postscript file then there is a shortcut. Just set the terminal and output, and do your plots! They will automatically be overlapped. For example,

gnuplot> set term postscript eps
gnuplot> set output 'overlap.ps'
gnuplot> plot x*x t "function"
gnuplot> set size 0.4,0.4    # next plot is 40% of the size
gnuplot> set origin 0.3,0.5  # and shifted 30% right and 50% up
gnuplot> plot 2*x t "derivative"
gnuplot> quit

will produce the following:

For more complicated graphs you'll want to modify this procedure a bit. Let's say you've got two (fullsize) graphs and you've saved them to command files a.plt and b.plt. To plot them overlapped you need to manually edit the files with a text editor and remove all lines containing set term ..., set output ..., set size ..., and set origin .... Then, in Gnuplot, type

gnuplot> set term postscript eps
gnuplot> set output 'overlap.ps'
gnuplot> set size 0.5,0.5  # set size and position of a
gnuplot> set origin 0,0.5
gnuplot> load 'a.plt'      # plot a
gnuplot> set size 0.5,0.5  # set size and position of b
gnuplot> set origin 0.5,0
gnuplot> load 'b.plt'      # plot b
gnuplot> quit

I don't know if this is good style for encapsulated postscript files, but it seems to work! The above procedure may also be handy for use with set multiplot in Gnuplot 3.6.

Problem: Sometimes, when I start up Gnuplot (win32 version) the text font is too small to read. Why is this and how can I fix it?

Solution: You are probably using a font which only accepts particular point sizes. If the selected size is not available (I think) it uses a lookup table to select the best replacement. The lookup table can get confused sometimes so it uses a tiny font. To fix this run Gnuplot and right-click in the command-line area. Select Choose Font... from the menu and pick a new font size that works for you. Don't forget to save your settings by right-clicking again and selecting Update wgnuplot.ini.

Problem: I use a particular command (such as fit) when plotting, but since it is not essential to the plot it doesn't get saved when I save the plot. How can I append it to the plot file?

Solution: There are a few ways of doing this (the easiest being to manually edit the plot file with a text editor) but I want to focus on one way in particular. If you are using the Windows version of Gnuplot you can manually edit the menu items by editing the file wgnuplot.mnu in Gnuplot's home folder. What you want to do is add the following lines wherever it's convenient (probably in the File menu):

	&Append last command
		{^P} >>"[SAVE]Append to[EOS]*.plt[EOS]"{^A}!command.com /c echo {ENTER}

1. asking for a plot filename to append to (eg. plot.plt),
2. jumping up to the previous command and typing >>"plot.plt" at the end of the line,
3. jumping to the beginning of the line and typing !command.com /c echo, and
4. hitting the Enter key.

The >>"plot.plt" string redirects the output to be appended to the file plot.plt. I had to call command.com /c because just calling echo alone didn't seem to support redirection.

Using this command would look like...

gnuplot> fit a*x+b 'data.dat' via a,b        (last command)
gnuplot> !command.com /c echo fit a*x+b 'data.dat' via a,b >>"plot.plt"

Problem: gnuplot 3.7.1 is supposed to be able to fit over a restricted range of y-values with the command fit [xlo:xhi] [ylo:yhi] ... but this feature is broken. Is there a workaround?

Solution: Of course there is. What I did was to define a set of functions,

gnuplot> limitlo(x,lo) = (x>lo) ? x : 0/0
gnuplot> limithi(x,hi) = (x<hi) ? x : 0/0
gnuplot> limitlohi(x,lo,hi) = limitlo(limithi(x,hi),lo)

gnuplot> ylo=-10; yhi=+10
gnuplot> fit [*:*] a+b*x 'data.dat' u 1:(limitlohi($2,ylo,yhi)) via a,b

Note that putting the limit function in the fitting function (eg. fit [*:*] limitlohi(a+b*x,ylo,yhi) 'data.dat' u 1:2 via a,b will not work, giving you an Undefined value during function evaluation error message.

Also note that ylo and yhi are only used once at the beginning of the routine so it won't work to make them depend on fit parameters. (In other words, fit [*:*] a+b*x 'data.dat' u 1:(limitlo($2,a)) via a,b won't do what you expect/hope.)