The ChartSeriesLabelDefinition class exposes the property Format
, which can be used to change the formatting of the label. In your case the code would probably look similar to this:
Of course, this will only work If the type of the underlying value is DateTime, but if your data is supposed to represent a point in time, DateTime seems to be a better choice of type than double. If your scenario requires the hours to be kept as doubles, you could convert them to DateTime before passing them to the chart.
I hope this solution is suitable for your scenario. Please, let us know if you have any concerns.