Need to be able to specify offset somehow:

* zero offset for continuous data
* zero offset for polar coordinates
* smaller offset as dimensions increase
* default to zero offset at bottom level?

Transformation function.

Automatic binning for continuous variables.

Look at Dykes mosaicplot labelling algorithm (big text)

Labels

* ability to specify which levels to use for labelling 
  (potential multiple levels)
* legend that displays margins in table format.
