Uses perceptualdiff to catch regressions / changes in charts as they are
displayed by a browser (in this case, phantomjs).
Currently has exemplaries for basic line, area, bar and stacked bar
charts.
- The options namespace should stay flat - no nested config values.
- Used single array literal instead of chained Array.concats.
- Added default values to config defaults, instead of using default
method arguments.
- Tweaked spec.