It sounds like you're looking for a heatmap type of overlay, not a 
choropleth map, given that you have a collection of coordinates.  If that 
is correct, then something like this might help.

df <- structure(list(long = c(-4.45, -5.04, -4.41, -4.7, -3.87, -2.67, 
-4, -4.37, -3, -3.04, -1.71, -1.26, -2.5, -3.42, -4.12, -2.88, 
-1.5, -0.88, -0.13, 1.32, 0.49, 0.07, -0.92, -2, -3.5, -4.45, 
-6.36, -6.32, -7.24), lat = c(58.12, 57.53, 57.34, 56.47, 57.08, 
57.32, 56.21, 55.43, 55.4, 54.6, 54.95, 54.2, 53.7, 52.92, 52.02, 
52, 52.8, 53.3, 52.54, 52.61, 51.9, 51.19, 51.81, 51.26, 50.84, 
50.48, 54.39, 54.91, 54.55), val = c(35.57, 30.63, 29.71, 55.05, 
28.26, 59.74, 6.04, 41.46, 53.67, 37.04, 81.06, 11.72, 24.34, 
63.98, 42.86, 64.28, 88.43, 79.16, 3.91, 27.34, 21.56, 89.71, 
69.43, 70.64, 64.56, 68.8, 32.05, 27.13, 85.86)), .Names = c("long", 
"lat", "val"), row.names = c(NA, -29L), class = "data.frame")

require(maps)
require(akima)

map("world", type="n", xlim=c(-7.82, 1.53), ylim=c(49.82, 59.04))
image(interp(df$long, df$lat, df$val, duplicate="median"), add=T)
points(df$long, df$lat)
map("world", add=T)

But, if you want a choropleth map for the UK or you don't want the heatmap 
extending beyond the country's boundaries, I can't help you.

Jean


Reza Salimi-Khorshidi <worx.of.mine@gmail.com> wrote on 08/25/2011 
12:48:07 PM:
> 
> Thanks Jean,
> but this package appears to be perfect for the US + a couple of 
> other country that unfortunately UK is not one of them. What I want 
> is a solution that takes a list of coordinates/postcodes/cities and 
> a list of values and gives me a colourful UK map. Any thoughts?
> Cheers

> On Thu, Aug 25, 2011 at 3:50 PM, Jean V Adams <jvadams@usgs.gov> wrote:
> 
> Check out the maps package for a choropleth map.  Specifically the 
> map() function (and its argument col=). 
> 
>      ?map 
> 
> Check out the image() function for a heat map which can be overlaid. 
> 
>      ?image 
> 
> Jean 
> 
> 
> Reza Salimi-Khorshidi wrote on 08/25/2011 09:00:04 AM:
> 
> > 
> > Hi all,
> > I would like to use R in order to generate a Heatmap/Choropleth 
overlaid on
> > the UK map. Can someone please help with finding the right 
package/code? Any
> > help is much appreciated.
> > Best, Reza
> > 
> >    [[alternative HTML version deleted]]
> > 
> > ______________________________________________
> > R-help@r-project.org mailing list
> > https://stat.ethz.ch/mailman/listinfo/r-help
> > PLEASE do read the posting guide 
http://www.R-project.org/posting-guide.html
> > and provide commented, minimal, self-contained, reproducible code.
> 

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