Rendering area in a line chart after adding data to the crossfilter is broken

Since the area path is selected with `.selectAll()`, an existing `<path>` node won't get it's `__data__` property set to the data of the parent node (the `<g>`). So when new data is added to the crossfilter and the chart is re-rendered, the area element still has a reference to old data and appears broken.

BTW, thanks for all your hard work, dc.js is awesome!