Update AxisBase.getDataByKey to return an object literal with arrays mapped to corresponding data keys when an array of keys is passed as an argument. The method still returns a single array when a key string is passed as an argument. CartesianSeries.setAreaData now has a helper method to allow for extending the class more easily. The setAreaData contains more common functionality while the new helper method (setCoords) contains more implementation specific functionality.
Update series-cartesian-tests to reflect setAreaData's newly created helper method.