[R-sig-Geo] How to Show Missing Data with fields package two.colors() function

[dkod]

I have been working on a global temperature anomaly map script to show
anomalies in 2x2 long/lat grid. With help from Roger Bivand and Andy South I
now have a script that makes the basic map with land surface background.

I use the fields package two.color() function to generate my color scheme. 

    my_scheme <- two.colors(n=256, start="blue", end="red", middle="white")

I choose this scheme so that "blue" reflects less than zero and "red"
reflects greater than 0 anomalies, with "white" for near zero values.

The overall map works as I hoped except that I can not figure out how to
handle missing data cells. I'd like them to show up as "grey". Currently
they are showing up as white along with the real near zero values.

How can I add a missing values color of grey to the map? Here's my overall
script.


# Load map related libraries
  library(fields)    # needed for image.plot()
  library(sp)        # needed for coordinates, SpatialPointsDataFrame
  library(maptools)  # neeed for wrld_simpl map

# Get world shape for map background
  data(wrld_simpl)            ## from maptools package
  shp <- wrld_simpl

# Read source data file 
 link <- "http://processtrends.com/Files/global_2x2_lota_data_latest.csv"
 rdf <- read.table(link, skip = 1, sep = ",", header=T)
 names(rdf) <- c("i", "j", "lon", "lat", "anom")

###############################
# Convert all anom data with 9999.0000 to NA
  rdf$anom[rdf$anom==9999.0000] <- NA         # convert all 9999.0000 to NA

## Promote to SpatialPointsDataFrame
  points_df <- rdf          # make copy of original file
  coordinates(points_df) = c("lon", "lat")     # convert to sp file with
lon/lat cordinates

## Promote to SpatialPixelsDataFrame
  pixel_df <- points_df
  gridded(pixel_df) <- TRUE

## Promote to SpatialGridDataFrame
  rdf_sp = as(pixel_df, "SpatialGridDataFrame")


## Establish color scheme
  my_scheme <- two.colors(n=256, start="blue", end="red", middle="white")

## Generate Plot
    par(mar=c(2.5,2,1,1)) ; par(oma=c(0,0,0,0))  # set plot par(mar=)
    g_plot <- as.image.SpatialGridDataFrame(rdf_sp["anom"])
    image.plot(g_plot, col=my_scheme,las=1,
      axes=T, horizontal=T,  ylim=c(-91,91), xlim=c(-181,181), zlim =
c(-6,6),
      legend.mar=4,legend.shrink=0.5)
   plot(shp, border="black", add=T)

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