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Try looking at setting the x-axis of the chart with a DateAxis. You can then work on getting the display you wish by setting the DateFormat for this object, formatting the said DateFormat the way you...

See the following class to use as an alternative to an XYSeries:
TimeSeries

I don't understand. XYSeries are supposed to be just that, with x and y numerical values. What sort of graph are you looking to create? Do you just want to change the X-axis display?