axisMaximum won't allow a dataSet that is smaller than it

* [x] I've read, understood, and done my best to follow the [*CONTRIBUTING guidelines](https://github.com/jjatie/Charts/blob/master/CONTRIBUTING.md).

## What did you do?

Set xAxis.axisMaximum to a higher value than the data set's max is on a line chart.

## What did you expect to happen?

I expect the graph to partially draw.

## What happened instead?

The chart would not draw similar to this issue.

_If there is another way to draw a chart with a partial dataset (ie. one with dates along the x-axis that have no y data yet) then I'm all ears!_