Null data points on categorical axes should be ignored

[etpinard]

See http://codepen.io/etpinard/pen/wMLEoO

At the moment, null data points on numerical axes are ignored whenever connectgaps is set to false. But, axes with type set to category stringify the null values as categories.

[mdtusz]

The same behaviour is there when using undefined - should we only be checking for null to leave a category gap, or do we want undefined to be excluded as well?

[etpinard]

We should exclude undefined values as well