displaying an error if gnuplot is not found

[John W. Eaton]

On 16-Apr-2008, Søren Hauberg wrote:

| It seems people don't read the top of the long error message that is
| presented if gnuplot is not found. At least, I think I've seen a bunch
| of questions on the help list that could simply be answered by "install
| gnuplot". Perhaps a patch like the attached could make this easier to
| figure out?

I applied this patch, but shortened the error message to

  error ("you must have gnuplot installed to display graphics; if you have gnuplot installed in a non-standard location, see the 'gnuplot_binary' function");

jwe