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I also have same problem when using LineSeries with CategoryAxis and LinearAxis.
It seem that when using LineSeries, X & Y axis must be in double.
My solution is to add dummy field in double into the model
Class DateValue {
double Value = 10.5, 30.7,10.3
string MonthName = "Jan", "Feb", "Mar"
double dummyField = 0,1,2 (must start with 0)

then in Category Axis, set LabelField to MonthName but in LineSeries, set DataFieldX = "dummyField"