Thank you for providing us with detailed information about your scenario. It helped us to identify the cause of your issue. The reported behavior is caused by the data you are using. Please keep in mind that in order to draw a line you need at least two points. In your business data the points that are defined are alternating with null. Meaning that every second point in your data is null. This means that the chart is not able to draw any line. If you change the LineSeries with a BarSeries you will see that fact.
In order to draw the lines you can change the null value with the average value of the points surrounding the null value. For example if you have the values 2, null and 4 you can replace the null with (4+2)/2=3 and the line will be drawn.
I hope this information will help you out.
Pavel R. Pavlov
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