Add bar chart to show transaction volumes over time

Add new bar chart module for transaction volumes over time. This is called the "bar_chart_with_number" module, since that is what it is. 

As part of this story: 

* Fix a few bugs in our graph and view code. 
* Refactor existing journey code to create new shared bar chart code, from which 
both the journey module and the new bar_chart_with_number module now inherit. 
* Create the new module. 
* Add tests for untested existing code. 

Henry has seen the completed module, and okayed it from a design point of view.

Pivotal story: https://www.pivotaltracker.com/s/projects/911874/stories/67393264

NB: I guess I'll also need to update cheapseats for these new modules: will create a 
separate pull request for that.