Add data point labels

[rdnetto]

This PR adds the ability to annotate scatter plots and bar graphs with labels. The supported options mirror the arguments of drawTextR, with some additional support for controlling position relative to the point/bar.

[timbod7]

Thanks for this PR.

Were you aware of the Graphics.Rendering.Chart.Plot.Annotation type? I would have expected that it would meet your needs for adding textual annotations to point and line plots.

The bar plots change looks valuable. However I would expect that the anchor values would be the same for each label, and hence could be passed once, rather than on each label. Also, Requiring the two parallel structures:

   _plot_bars_values          :: [ (x,[y]) ],
   _plot_bars_labels          :: [[BarLabel]]

feels awkward to me. I think it would be cleaner to just include the labels directly, ie

   _plot_bars_values          :: [ (x,[(y,String)]) ],

[rdnetto]

I was completely unaware of the Annotations module - I've removed the code it duplicated and updated one of the tests/examples to demonstrate it.

I've modified the bar labels code as suggested, and defined a lens to preserve compatibility. It's not quite law abiding, since it always sets the labels to empty strings, but I couldn't think of a better way of handling it.

[timbod7]

Thanks - I've taken a quick look, and it looks good now.

I'll review in detail in the next couple of days.

[timbod7]

Fixed by #253