new boolean property on linechart to toggle mouseover dots

[davisford]

So, on line charts, this feature:

i.e. the x/y tracking to the axes, and the dot that tracks the line on mouseover is something that is currently not optional. It is always generated. I had a need to build something else for my line charts...namely a tracker that shows up not just if you mouseover on the line but on the whole chart, and also a linear regression slope, and I wanted a larger dot to make it more visible.

Seeing as how my linechart features collided with the built-in features, I wanted to make the built-in features optional, so I added a new property on linechart:

linechart.xyTipsOn(true); // the default
linechart.xyTipsOn(false); // disable the tracking dot and xy refs

By default, the behavior is still on, so it should affect nothing. This just gives me the ability to disable it to inject my own behavior that doesn't collide with it.

Added documentation for the new public API, as well as a few unit tests.