Add stacked chart for transit length indicator

Filters indicator endpoint to only return system/mode indicators

Refactors some of the stuff in data-controller to data-service
and attempts to clean up the chart definition objects.

See notes in data-partial.html regarding use of ng-init
to temporarily store data transformations.
Merge pull request #207 from WorldBank-Transport/feature/stacked-chart

Add stacked chart for transit length indicator