numeric vector of length 2 giving the range of the underlying
data
breaks
either a vector of break values, or a break function that
will make a vector of breaks when given the range of the data
labels
either a vector of labels (character vector or list of
expression) or a format function that will make a vector of labels when
called with a vector of breaks. Labels can only be specified manually if
breaks are - it is extremely dangerous to supply labels if you don't know
what the breaks will be.