V2.0 dev skip null fixes

[tannerlinsley]

This fixes bad values from killing the chart. NaN, undefined, and null are now all treated the same.

skipNull and drawNull are no longer options in the line element. There weren’t used much, because quite frankly, they didn’t work! Now it’s all automagical, and bad values won’t be drawn.

You can still manually skip a point by marking that point’s _model with a skip: true property.

[etimberg]

Is it ok to lose the return?

[tannerlinsley]

Yeah

[etimberg]

Sounds good. As soon as tests pass, this is good to merge 😄

[emcniece]

I'm curious about the skipNull feature, especially now that I need it ;)

My use case: line chart plotting multiple series, dates on the x-axis. Each series may or may not have a point on a given date. Instead of having the line interrupt, I would prefer to have it join up to the next point!

I am currently using Chart.js v2.1.3 in conjunction with http://jtblin.github.io/angular-chart.js/

[emcniece]

So far I have managed to extend the line chart controller and am able to set point._model.skip=true for given points, but this results in the same behaviour that currently exists - the series breaks, then continues at the next point. (duh, that's what this PR is all about)

What can we do to make the line continuous? I'm currently thinking about averaging the surrounding points, plotting a fake point in between (with the averaged values) and hiding the point with radius=0. Any other suggestions?

[deanshub]

As of #2721 you can add the spanGaps attribute which allows this skipping ability as described here