add preprocessData step to coordinateGridChart

allows subclasses to split data, create children, etc.
allow rerender of legend

erase old svg g.dc-legend element on legend render
various fixes for series chart

use new preprocessData hook instead of plotData
default to data format of crossfilter of pair keys
presort the data (unless disabled) because crossfilter sorts tuple keys wrong
redraw legend if series added/removed
don't send anchorname to child constructor (not used)
improved removal of disappearing series
more intensive test of series chart

buttons to click between different data with more and less series
use a crossfilter with pair keys instead of setting data directly
more complex chart prototype (line chart with interpolated curves)
Merge pull request #352
call .data with chart context
support non pivot series carts