Allow axis labels to be null

[kepano]

For many charts I find that the axis label is redundant, since it is obvious from the data or the title what the chart is measuring.

I could not find a way to hide the axis label such that the chart properly resizes with the viewbox. For example, if I set the axis labels to transparent, there is a gap.

Proposed solution, could be one of the following:

1. Allow xAxisLabel: null and yAxisLabel: null
2. Add showXAxis: false and showYAxis: false

[kepano]

I just found a hacky way around this. A little strange because it does require a space between the quotes

    xAxisLabel: ' '
    yAxisLabel: ' '

[pyrochlore]

The second way you proposed looks good. Will do.