Gnuplot is a portable command-line driven graphing utility for Linux, OS/2, MS Windows, OSX, VMS, and many other platforms.

Add the following lines to ~/.config/geany/filetype_extensions.conf under the respective sections [Extensions] and [Groups]

[Extensions]
Gnuplot=*.gp;*.gnu;
[Groups]
Script=Gnuplot;

Save the configuration file below in your Geany filetype definition files config directory, e.g. ~/.config/geany/filedefs/filetypes.Gnuplot.conf

NB: Note that this filetype definition file is in version 0.1, meaning it can be used but needs improvement.

filetypes.Gnuplot.conf

# For complete documentation of this file, please see Geany's main documentation
 
# Geany Color Syntax support for Gnuplot 4.2+
# version 0.1
# April 5, 2013
 
# This is filetypes.nsis edited to support the Gnuplot Programming Language
# copy this file, filetypes.Gnuplot.conf, into the directory ~/.config/geany/filedefs/
 
[styling]
# Edit these in the colorscheme .conf file instead
default=default
comment=comment_line
stringdq=string_1
stringlq=string_2
stringrq=string_2
function=keyword_1
variable=keyword_1,bold
label=label
userdefined=keyword_2,bold
sectiondef=keyword_1
subsectiondef=keyword_1
ifdefinedef=keyword_1
macrodef=keyword_1
stringvar=string_1
number=number_1
sectiongroup=keyword_1
pageex=keyword_1
functiondef=keyword_1
commentbox=comment
 
[keywords]
# all items must be in one line
variables=using with linecolor lc linetype lt linewidth lw linestyle ls line at cos sin tan sum via errorbars xerrorbars yerrorbars xyerrorbars pointtype pointsize 
functions=plot replot print set unset reset fit
lables=terminal output title samples style label xlabel ylabel xrange yrange grid border datafile multiplot origin size key logscale
userdefined=rgb postscript eps lines
 
[lexer_properties]
nsis.uservars=1
nsis.ignorecase=1
 
[settings]
# default extension used when saving files
extension=gp
 
# single comments, like # in this file
comment_single=#
# multiline comments
#comment_open=
#comment_close=
 
 
# This setting works only for single line comments
comment_use_indent=true
 
# context action command (please see Geany's main documentation for details)
context_action_cmd=
 
# lexer filetype should be an existing lexer that does not use lexer_filetype itself
lexer_filetype=NSIS
 
[build-menu]
EX_00_LB=View EPS File
EX_00_CM=evince %e.eps
EX_00_WD=
FT_00_LB=Compile Output File
FT_00_CM=gnuplot %f
FT_00_WD=