[R] mean of runoff for several years

[dominique]

Hello everyone
I have a dataframe with 4 colums (year, month, day, runoff) for 1993-2009.
Now I like to calculate the average runoff for each day. Finally I like to
plot the median runoff for all this years. 
I tried with some loops, but it didn't work. Do you have any Tips for my
problem?
Any help is greatly appreciated! Thank you very much.
Dominique



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