[enhancement] equal position_dodge for incomplete factor combinatios

[DSLituiev]

It has been often asked on stackoverflow and elsewhere how to plot bars /columns/ boxplots of even width when grouped by x and fill with some factor combinations missing, e.g. in following figure one wishes to have all bars of the same width:

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http://stackoverflow.com/questions/11020437/consistent-width-for-geom-bar-in-the-event-of-missing-data
http://stackoverflow.com/questions/15367762/include-space-for-missing-factor-level-used-in-fill-aesthetics-in-geom-boxplot?noredirect=1&lq=1
http://stackoverflow.com/questions/41968652/changing-geom-bar-widths-on-ggplot?noredirect=1&lq=1
http://stackoverflow.com/questions/38101512/the-same-width-of-the-bars-in-geom-barposition-dodge
http://howtodevelop.eu/question/r-ggplot2-changing-geom-bar-widths-on-ggplot,135411

etc, etc...

Some of recipes suggested work pretty well, but I wonder if this should be a default behaviour or at least some of these recipes can be incorporated as, let us say position_equal_dodge option?

[hadley]

I implemented this 10 days ago 😄

#1935