[R] plotting x y z data from an irregular grid

[erika]

Hi,
I'm trying to plot some data (z) that is linked to lat&long coordinates
(x&y). These coordinates are not however on a regular grid. I also have some
shapefiles on which I would like to overlay the data. I can plot the
shapefiles (country/city outlines) and overplot the data, but only using
quilt.plot because I otherwise always get the error message that 
'Error in image.default(..., add = add, col = col) : 
  increasing 'x' and 'y' values expected' 
which has something to do with the organization of my data but I cannot
figure out how to change it correctly. This is the code that I have that
works:


data<-read.csv('with coord_observational log data trends all years all
data.8.11.10.csv', header=TRUE)

## this is what the 'data' looks like:
head(data)
  X SiteCode Latitude   Longitude           p perc_per_year
perc_per_year_lower
1 1      A30 51.37357 -0.29172504 0.369164267    -0.4781589          
-1.390382
2 2      BB1 51.68299 -0.03254972 0.005546354    -3.1810064          
-5.665312
3 3      BG1 51.56375  0.17789100 0.000405606    -3.2260763          
-5.344999
4 4      BG2 51.52939  0.13285700 0.434756172    -5.1558318         
-22.123800
5 5      BH1 50.82334 -0.13724510 0.183375348    -0.8735160          
-2.240289
6 6      BH2 50.82785 -0.17051300 0.002702969    -2.1443157          
-3.543378
  perc_per_year_upper         sig
1           0.4786508 -999.000000
2          -0.8588293   -3.181006
3          -1.5251377   -3.226076
4          11.0957982 -999.000000
5           0.3431518 -999.000000
6          -0.7966338   -2.144316

#read in the shapefile
england<-readShapePoly('D:/arcGIS/england boundary/england.shp')
class(england)
#define the projection
proj4string(england)<-CRS('+proj=tmerc +lat_0=49 +lon_0=-2 +k=0.999601271625
+x_0=400000 +y_0=-100000 +ellps=airy +units=m +no_defs')
# transform the map into the WGS84 projection (epsg:4326):
england.wgs<-spTransform(england, CRS('+init=epsg:4326'))

plot(england.wgs)
#plot data over the map:
quilt.plot(data$Longitude, data$Latitude, data$perc_per_year, add=TRUE)

My problem is that I would like to be able to change how this data looks
(not just 'grid squares', but circles, etc) which I can't seem to do wtih
quilt.plot. If I could do this I could plot one layer of gridded data
(squares as in quilt.plot) and overlay a second layer of 'z' data as points.
I have tried plot.surface and image.plot and a number of others, but because
of the error message that I get above it won't work. I can use image.plot,
etc if I create a grid and interpolate my data onto the grid (see code
below), but I don't want interpolated data, I would like discreet values for
each lat&long. 


x<-seq(-4,2, by=0.0625)
y<-seq(50,53, by=0.0625)
d1<-expand.grid(x=x, y=y)
data.li<-interp(data$Longitude, data$Latitude, data$perc_per_year,
duplicate='mean')

So my questions are, (1) is there a different function that I should be
using with my data as it is and still be able to overplot it onto the map
that i've plotted? or (2) is there a way to create this grid and integrate
my data into the grid, but not interpolate it?

Any help would be very much appreciated. My R skills just are not good
enough to do this yet.

Thank you!!
Erika. 



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