Add a background to the column charts
Add a background to the column charts

* Add custom initialize method to the column chart to place the data
  layer behind the axes
* Add a .bg rect to the column chart

SeedScientific/polio,unicef/rhizome,unicef/rhizome,unicef/rhizome,SeedScientific/polio,unicef/polio,unicef/polio,unicef/polio,SeedScientific/polio,unicef/rhizome,unicef/polio,SeedScientific/polio,SeedScientific/polio