Fix x-axis label rendering partially off the side of the chart when it is too long

I have found that when an time-axis label doesn't fit between its graticule and the edge of the chart, the label dangles off the side of the chart. Here is an illustration:

![Before](http://people.apache.org/~mbooth/before_fixed_chart.png)

This commit enhances the time axis so that it does not render the label if it will not fit on the chart:

![After](http://people.apache.org/~mbooth/after_fixed_chart.png)

Note that it removes the label after adding (instead of preventing it from being added) because I have read that you cannot reliably find an element's width before it is added to the DOM.