First you will need to set the ItemsSource of the series to a collection of items that have a property of type DateTime.
Then you have to set the CаtegoryBinding of the series to the DateTime property. If the CategoryBinding is not set, the chart automatically sets the categories of the data points to 1, 2, 3...
I have attached a sample project to demonstrate how you can setup the chart in order to display the categories properly.
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