[R] ggplots - how to increase the number of levels

[Ptit_Bleu]

Hi,

Thanks to Hadley Wickham, I am now able to plot some nice "weather" graphs
(see 
http://www.nabble.com/how-to-plot-image%28%29-without-painting-a-map-%28the-background%29-td15546906.html#a15584405
How to plot a graph on a map ).
For some parameters, I would like to increase the number of levels (the
number of grey tinges in fact ("nuances" in french - not sure "tinges" is
the correct english translation)).
I tried to add nlevels=10 to the following command :

p <- qplot(data=lonlat1, longitude, latitude, geom="tile", fill=TCDC,
xlab="", ylab="") + 
scale_fill_gradient(low="white", high="grey") + geom_contour(aes(z = TCDC))
+ france_map

... but nothings changes.
I have always 5 levels for TCDC : 20 / 40 / 60 / 80 /100.
http://www.nabble.com/file/p16396028/tcdc_map.jpg 


Could you please tell me how to solve this easy (except for me) problem ?
Have a nice week,
Ptit Bleu.
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