I'm very new to R, so please forgive me if I just missed the answer in
existing documentation...

I have a data set with at least three columns, X, Y, and Z.

I want to produce a chart where one axis shows all the unique values of X,
and the other axis shows all the unique values of Y.  Each cell within the
chart should contain the result of applying an aggregate function (such as
mean(), for example) to Z value of those rows that are associated with that
cell (via their X and Y values).

Can someone recommend a good way to do this?

Thanks very much,
Christian

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