[R] table of means/medians across bins used for a histogram

[lalitha viswanath]

Hi
I am trying to get a table of means of parameter 1
across BINS of parameter 2.

I am working in proteomics and a sample of my data is
as follows

cluster-age clock-rate(evolutionary rate) scopclass
0.002       10                              A
0.045       0.1                             B
0.13        15                              A 
0.15        34                              D
....
....
.... 
....

Scop class has only 9 distinct categories (A-I)
Whereas cluster-age and clock-rate are discrete
variables greater than 0.

I am trying to do two things with this kind of data,
out of which I managed to accomplish one thanks to the
documentation and pre-existing queries on the mailing
lists.
1. Plot a histogram of the age distribution with scop
class category superimposed on each bin. I managed to
do this with barplot2. 
2. Now I am trying to plot a scatter plot of the age
v/s the clock-rate. However to eliminate possible
sampling errors, we are trying to get an average of
the clock-rate for each of the bins used above. 
i.e. before plotting a x-y plot, i wish to compute
average clock-rate in each of the bins for the age and
then plot a x-y plot of the age v/s clock rate.

Can anyone point me to appropriate functions for the
same?
I am trying to work with prop.table, cut, break, etc.
But I am not heading anywhere.

Thanks
Lalitha