* variables are grouped by plotter type before being processed by graph. So plotters should expect a list of variables now.
sided bar graphs implemented, cumulated is temporary broken
* bar graph handle both sided and cumulated bars
* also horizontal bar graphs handle sided and cumulates correctly
* refatoring: common code between Bar and Barh moved to AbstractBar and added template methods to handle differences
* automatic x_tick_labels orientation restored and improved
* cleanup of unused code