[R] intensity plot

[paladini at trustindata.de]

Hello,
I have got a data frame looking like this:

Country          Sector    m-value

USA              Banks     38.5
USA              Media     17
USA              hospitals 2.3
Germany          Banks      56
Germany          real estate 1
Italy            Banks      34
Italy            Media      23
Italy            real estate 78
Italy            Beverage    23
....


I would like to have a graph, let's say country on the x-axis, sector  
at the y-axis,  and for
each compination of country and sector a square in blue. The shade of  
the color should depend on the
level of the m-value.(This graph should look more or less like the  
graph when you use corrgram() to display correlation)

I tried ggplot() but I didn't get far. So it would be really nice if  
somebody could help me.

Best regards

Claudia