Chart.js' scatter line charts care about data point ordering

[marianogappa]

The following dataset

2 4
1 3
4 4
3 3

Yields the following scatter line chart

While this behaviour is very flexible, I believe anyone wanting to plot a scatter line chart would expect the x values to be sorted, and this should be the default behaviour.

The aforementioned issue also happens when the x axis is composed of time-based data points.

[marianogappa]

After the above commits, the scatter line chart looks like this: