[R] Bin Category Labels on Axis

[jproville]

Hi, 

I'd really appreciate if someone could give me some help or advice about
this - I've tried everything I know and am clueless about how to proceed!

I've written a script to import ASCII data of raster maps, bin them into
categories, calculate the mean values within these bins and plot the two in
a simple graph. I'm running into problems with my x axis, as R cannot add
the bin categories as labels and simply outputs the factor values of the
number of bins. 

In much more detail:
One set of data is fractional values for the amount of cropland in a
designated area (raster cell). The second is annual average temperature
data, in the same format. I import these using:
xRaw<-scan(".../Temp.txt")
yRaw<-scan(".../Cropland.txt")

and then designate cells with values of -9999 as NAs:
x<-ifelse(xRaw==-9999,NA,xRaw)
y<-ifelse(yRaw==-9999,NA,yRaw)

Then, I compile them into a 2 column data frame, and exclude the NA values:
zRaw<-data.frame(x,y)
z<-na.exclude(zRaw)

I am then left with data frame 'z' which is a refined list of data. Using
the 'cut' function, I assign each row into one of 69 different bins:
z$bins<-cut(z$x, breaks=c(-36:33), include.lowest=TRUE)

Each bin is now 1 degree Celsius wide. Within each one, I calculate the mean
value of the corresponding fractional cropland data:
a<-tapply(z$y, z$bins, mean) 

Object 'a' now contains each bin category and the associated mean cropland
values, so 69 points in total. 


Now, a problem arises when I try and plot these values using:
plot(a, xlab="Temperature", ylab="Fraction of Cropland", pch=20)
What happens is that R intuitively labels my x-axis from 1-70, which
corresponds to the bin factor values that were returned when the 'cut'
function was used. I would like to simply label the x-axis with my
temperature values, from -36 degrees to 33 degrees, instead of having
numbers 1-69. How can I do this?? 

Thanks,

Jeremy Proville
M.Sc. Candidate
Bioresource Engineering, McGill University
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