Created nv.models.tooltip in tooltip.js.
This new model encapsulates most of the tooltip functionality.
Updated nv.models.tooltip to have a custom HTML content generator. The default is to put tooltip inside a <table> tag.
Updated scatter.js so that when you mouseover a path-point, it will return all series information in the mouse event dispatch.
Updated nv.models.tooltip with more features.
Updated styling of the tooltip.
Added the 'highlightPoint' dispatch on nv.models.scatter. This dispatch lets the programmer highlight any point on any series.
Currently used in lineChart to highlight points.
Added color legend square to the tooltip for each series.
Added 'ClearHighlights' dispatch to scatter.
Created preliminary implementation of mousemove event on lineChart.
Added interactiveLayer.js and integrated this utility into lineChart.js.
Added test page for lineChart. Continued work on making the interactive line layer.
fixed discrete bar chart y bleeding on ydomain min >0
fixed bug on previous push
Developed stronger backward compatibility with the old tooltip format. Old
tooltips work by default, but if you enable 'useInteractiveGuideline', the
new stuff will show.
Created nv.interactiveBisect utility function, and integrated it into the
interactiveGuideline logic.  This function is used to find the x point
position given a mouseX location. It's a more robust solution. Made sure
chart tests passed.
Added six more important line chart test cases. Changed tests to include
area charts.
Fixed tooltip display edge case, where if the tooltip is very large, it
will render off window if too close to the bottom.
Cleaned up interactive guideline dispatch code.
Integrated interactive guideline into cumulativeLineChart and updated
example.
Integrated interactive guideline into stacked area charts. Updated the
stacked area example.
Created new general stylesheet for the nvd3 chart test pages.
Added test page for stacked area chart.
Adding unit test on lineChartTest.html for when points are NaN, undefined,
etc.
Found a few edge cases where errors happen. Creating nv.utils.NaNtoZero()
function, to be used in places where points and lines are rendered.

Using NantoZero in scatter.js and interactiveGuideline.
Fixing issue in scatterPlusLineChart where tooltips don't activate on
chart initial load.
Turning off Voronoi point path creation, if using the new interactive
guideline and tooltip.

Turns out, Voronoi is not needed in this new interactive mode!