rplot: style, stats
rplot: progressively reduce alpha for filled plots
rplot: set viewport (yrange)
rplot: handle canvas internally
rplot: set default grid
rplot: new feature, stacks

- Allow stacking histograms instead of overlays by setting the
  Rplot.stack property to True.
- Rename variables appropriately
  plottable (hist/graph) ⊂ plot (1 spot on grid) ⊂ plots (whole grid)
rplot: add test (by eye)

FIXME: how to unit test this?
rplot: update status in README
rplot: clean-up __init__