[R] shrink a dataframe for plotting

[Alexy Khrabrov]

I get tables with millions of rows.  For plotting to a screen-size  
jpg, obviously just about 1000 points are enough.  Instead of feeding  
plot() the original millions of rows, I'd rather shrink the original  
dataframe, using some kind of the following interpolation:

-- split dataframe into chunks of N rows each, e.g. 1000 rows each
-- compute average for each column
-- issue one new row of those averages into the shrunk result

Is there any existing package to do that in R?  Otherwise, which R  
idioms are most effective to achieve that?

Cheers,
Alexy