One of my projects is to overhaul the code for nv.models.legend.
Right now, we have duplicated code in all of our charts
for handling legendClick and legendDblClick. It turns out we
can handle this all in legend.js, and reduce the amount of
code in each chart model.

So far, I've converted lineChart.js to this new format.
[legend.js] added radioButtonMode. If set to true, chart
legend will behave like a radio button group when clicked.
Mentioned in issue #168.
[stackedAreaChart] Adding legend stateChange listener and removing old
legend click code.
Converting cumulativeLineChart to use legend stateChange dispatch.
Updated lineChart and stackedArea to update chart.state
whenever legend.stateChange is dispatched.
Updating almost all main chart types to use new legend.stateChange
code. All have been tested.
Updating scatterPlusLineChart.js to use legend.stateChange.
Updating README.md to highlight issue #168, legend.radioButtonMode().