Ggplot

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* renamed scale_manual to scale_identity to map position_identity and
stat_identity

* ensured all grob consistently named

* renamed aesthetics argument to mapping to be consistent with description in
book

* added useful utilities for modifying grobs

* bug fixes to x axis range with interval geoms

* added ability to turn legend off for single scale (currently undocumented)

* added economics data set and matching examples

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* complete rewrite of ggplot code base

* id/grouping completely rewritten so that automatically grouped when any of
the aesthetics is a categorical variable.This behaviour is defined in the
Grammar of Graphics, and makes things like qplot(wt, data=mtcars,
geom="density", colour=factor(cyl)) work nicely