d3-style syntax

[deanmalmgren]

This came up previously and will make more sense to do when we split catcorr across multiple files. It would be nice to have more modular functionality to make it easy to split out different functional units, particularly for testing purposes.

var cc = catcorr.data(data)
    .options(options)
    .render(selector, options)

var choices = cc.questions[0].choices()

// get the histograms when something is selected
var histograms = cc.questions[0].select(choices[0]).histograms();