Refactor last year chart

Pulled some instance variable assignment into the controller.

Also extracted a method regarding interpolated values for the present
month.
Shifting more instance variables to the controller

Also removing some instance variables that were never invoked in the
views. This sets up a future refactoring of the method of comptuting
rolling averages, which differs between the two types of "last year"
views being refactored.
Merge pull request #191 from doncruse/master

Refactor last year chart