show directionality on the arcs

[schnee]

See http://bl.ocks.org/rveciana/10668944

We'll want to apply a gradient (optional? I know that I want it there all the time, but others may want options) from a start color to an end color. I suppose we could interpolate between a vector of any number of colors, but let's keep that as a further enhancement.

[schnee]

we could do a color gradient, or we could do something like svg line markers:

http://www.svgbasics.com/markers.html

[DataStrategist]

+1. this would be great. Or directionality could also be controlled by setting stroke-width to different values so that the connector "points" so to speak.