Fixes #30 - Basic Plot "Label-less" Layers

Fixes an issue where multi-series basic plots would not render correctly unless each of the layers was provided with a label. Simplified the process by simply keeping a reference to the categorical index on each layer object when setting initial data for a plot (see `Epoch.Chart.Base.setData`). To fix each of the plots we simply use the category index as opposed to label when grouping data in a series with d3.