Re: geographic plotting

[Allin C. <cot...@wf...>]

On Fri, 6 Mar 2020, Ethan A Merritt wrote:

[a propos Manfred Schwarb's suggestion of using GeoJSON files
for geographic plotting in gnuplot]

> Good call.  Geojson looks quite promising.
> A real parser should be easy, but even a quick pass through sed
> or a text editor makes it usable.
> I have posted a proof-of-principle example using US state boundary
> data from a sample file on the geojson development site:
>
>  http://skuld.bmsc.washington.edu/people/merritt/gnuplot/

Thanks for the example, Ethan.

I've recently returned to this issue, exploring the shapefiles 
option. I've written some relevant utilities in C, designed to link 
against Frank Warmerdam's lipshape. (I'll be happy to share these 
once they're better tested.) With their help I've managed to create 
quite a nice map of the US states, colorized by state population 
levels. See 
http://ricardo.ecn.wfu.edu/pub/gretl/datamaps/statepop.png .

But the method I'm using for the colorization is clunky and I'm sure 
it can be improved upon. Here's my input file:

<gnuplot>
set term pngcairo size 660,400
set output 'statepop.png'
set linetype 1 lc rgb "white" lw 1

set palette model HSV file 'statepop.hsv'
set cbrange [0:50]
unset colorbox

set xrange [95:210]
set yrange [-175:-135]
set noxtics
set noytics
set border 0
unset key

plot for [i=0:*] 'merc.dat' index i with filledcurves \
  fc palette cb i, 'merc.dat' with lines lt 1
</gnuplot>

In the above, 'merc.dat' contains the state outlines from the .shp
file. The population data get into the picture via the pre-processed
'statepop.hsv'. To produce this (with its shades of blue) I ran a
loop across the states using H = 229/360, V = 93/100, and S[i] =
population of state i divided by the max population value.

Any suggestions on the best way to do the colorization within 
gnuplot? And also, perhaps, how to discretize it (by decile for 
instance)? Is there a way to replace

   fc palette cb i

with something like

   fc palette cb <function of i and the data to be colorized>

?

Allin Cottrell