Try to infer type of X axis data

When using `Chart.Line` with date for the X axis, the chart previously displayed some random numbers (well, DateTime converted to float) for X values. This change tries to infer the type from values (if we can access them) - if the values represent single day, we infer `Time`, if they represent dates, we infer `Date`, otherwise we return `DateTime`.

The result is that the X axis will be generally annotated with something usable!