OK, so I've looked at a few of the code samples and my biggest recommendation is that you include some higher level functions which can be customized. Compare the very simple histogram on the front page and the code used to make it with the code to make a histogram in R. Graphics is something R does right. The equivalent code in R:
C2 isn't a charting library; it's a collection of composable primitives for building statistical graphics.
There are some built in helpers---scales, axis templates, and the tick-selection algorithm---that would make it pretty easy to build exactly the histogram graphic you want.
If you want to pick from a menu of fixed data graphics, though, HighCharts, R's ggplot, or Tableau are much better choices than C2.
Take another look at ggplot2; it's exactly the "collection of composable primitives for building statistical graphs" that you mention; but it also gives you some higher level functions that give you a starting point that you can then customize.
There's a benefit to working in Hadley's / Wilkinson's grammar, but the tradeoff is that it obscures the underlying DOM representation (or R's graphics subsystem, as the case may be).
ggplot is for analysts, and c2 is for web devs and hackers.
>> vals = read.csv('somedata.csv')
>> hist(vals)