See NEWS for changes in latest version

ggplot 0.5.6
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Improved error messages and other notifications:
  * all geoms and position adjustments should now give an informative error message when required aesthetics are missing
  * better error messages if data not a data frame, or mapping not created by aes or aes_string
  * better errors for qplot when variables missing or data invalid
  * better error if somehow you are missing necessary scales
  * stat_bin informs you of the default choice of binwidth
  * stat_smooth gives helpful error messages for common problems
  * printing a geom now displays the data set that it uses (if not the default)

Other improvements:
  * colour and fill legends now surround by background plot colour
  * can now draw arrow heads with geom_segment, and have added an example demonstrating drawing a vector field
  * density plots should always include 0 on y axis
  * default boxplot outlier changed colour to black
  * stat_smooth supports categorical variables a little better
  * implemented hash methods for all ggplot objects.  This is the first step in making it easier for me to compare all examples between versions for quality control purposes

New data:
  * seals, contributed by David Brillinger and Charlotte Wickham, used for vector field example

Bug fixes:
  * geoms hline, vline and abline now all work correctly when a grouping variable is used
  * block histograms (where individuals are identifiable) now work correctly  
  * all ggplot objects should now print properly from the command line
  * fixed bug in geom_path when only 1 point
  * segments geom now works correctly for more coordinate systems
  * order variables in scatterplot matrix by order of variables in data.frame
  * geom_density deals with missing values correctly when displaying scaled densities
  * fixed bug in calculating categorical ranges
  * fixed bug in drawing error bars

Subtractions
  * now relies on R 2.6
  * removed grid.gedit and grid.gremove, and code replaced by grid.ls

ggplot 0.5.5
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Improvements:
  * ggplot now gives rather more helpful errors if you have misspecified a variable name in the aesthetic mapping
  * changed default hline and vline intercepts to 0
  * added "count" output variable from stat_density for creating stacked/conditional density plots
  * added parameters to geom_boxplot to control appearance of outlying points
  * overriding aesthetics with fixed values that have already been set with aesthetics now actually works
  * slightly better names for xaxis and yaxis grobs
  * added aes_string function to make it easier to construction aesthetic mapping specifications in functions
  * continuous scales now have labels argument so that you can manually specify labels if desired
  * stat_density now calculates densities on a common grid across groups.  This means that position_fill and position_stack now work properly
  * if numeric, legend labels right aligned
  * polar coordinates much improved, and with better examples

Documentation:
  * fixed argument documentation for qplot
  * added (very) rudimentary documentation about what functions return
  * documentation now lists extra variables created by statistics

Bug fixes:
  * coord_flip now works with segment and all interval geoms
  * geom_errorbar now works in all coordinate systems
  * derived y axes (eg. on histogram) are now labelled correctly
  * fixed bug in stat_quantile caused by new output format from predict.rq
  * fixed bug if x or y are constant
  * fixed bug in histogram where sometimes lowest bar was omitted
  * fixed bug in stat_qq which prevent setting aesthetics
  * fixed bug in qplot(..., geom="density", position="identity")
  * fixed stat_qq so that unnecessary arguments are no longer passed to the distribution function

Subtractions:
  * removed grid argument from ggsave, replaced by ggtheme(theme_bw)
  * removed add argument from qplot


ggplot 0.5.4
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  * border now drawn on top of geoms, instead of below - this results in better appearance when adjusting scale limits
  * ggplot() + aes() now modifies existing default aesthetic mapping, rather than overwriting
  * polish examples in facet_grid

ggplot 0.5.3
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  * added experimental scatterplot matrix, see ?plotmatrix
  * added new border.colour and grid.minor.colour options for better control over plot apperance
  * updated theme_bw to do better when drawing a plot with white background 
  * better default colour choices for gradients (and more discussion in examples)
  * fixed bug in ScaleGradient2 where scales with different positive and negative ranges were not scaled correctly
  * allow expressions as result from strip.text
  * fixed rare bug in geom_vline and geom_hline
  * fixed example in geom_abline
  * tweaked display of multiline axis labels

ggplot 0.5.2
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  * add argument to position dodge so it's now possible to accurately dodge things with different widths to their physical widths 
  * added median summary
  * new examples:
    * logistic regression example in stat_smooth
  * bugs fixed:
    * evaluation of arguments to layer is no longer delayed
    * can use categorical xseq with stat_smooth
    * x and y axes named incorrectly (thanks to Dieter Menne for spotting this)
    * can now pass position objects to qplot
    * y jitter calculated correctly, and jittered data rescales axis now
    * removed silly legend from quantile plot
    * extra arguments not being passed on to geoms/stats
    * fixed bug in stat_summary when summarising a factor
    * fixed bugs in stat_summary, geom_ribbon, and coord_trans examples

ggplot 0.5.1
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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

ggplot2 0.5
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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
