Re: geographic plotting

[Ethan A M. <me...@uw...>]

On Tuesday, 3 March 2020 14:30:40 PST Allin Cottrell wrote:
> This question has been posed before (a few years back) but I'm 
> hoping someone may be able to point me to the state-of-the-art,
> if there is such.
> 
> I'd like to be able to produce via gnuplot a map of (for example) 
> the USA showing the states colored by median income, or PM2.5 
> particulate emissions, or whatever.
> 
> I understand that the starting point would be a suitable US states 
> shapefile (I know where I can get that), which would have to be 
> translated to a format (resembling world.dat, I suppose) that can be 
> understood by gnuplot. I have some notion of how that might be done.
> 
> The next issue would be to get gnuplot to colorize specified areas 
> of the map according to the values of some additional data. 
> Unfortunately I have little notion of how to do that.

The development version of gnuplot supports 

	splot <data> with polygons

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
gnuplot> help polygons

      splot DATA {using x:y:z} with polygons
            {fillstyle <fillstyle spec>}
            {fillcolor <colorspec>}
 `splot with polygons` uses pm3d to render individual triangles, quadrangles,
 and larger polygons in 3D.  These may be facets of a 3D surface or isolated
 shapes. The code assumes that the vertices lie in a plane.
 Vertices defining individual polygons are read from successive records of the
 input file.  A blank line separates one polygon from the next.

 The fill style and color may be specified in the splot command, otherwise the
 global fillstyle from `set style fill` is used.  Due to limitations in the
 pm3d code, a single border line style from `set pm3d border` is applied to all
 polygons.  This restriction may be removed in a later gnuplot version.
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% 

This plot mode will be in version 5.4 (release candidate coming soonish).
Meanwhile you can build from the source for either branch:
  master
  branch-5-4-stable

I realize you don't need 3D, but plotting in projection is easy, and you even
have the option to map it onto a globe :-) 

I am uncertain whether there is adequate flexibility to use the 3rd coordinate
as a color flag.  If there isn't, there probably should be so let me know and 
I'll work on it.    

	Ethan