The reason why the line series doesn't show is that the bar series have Stack100 set as CombineMode. This combine mode automatically calculates the values of the bars as percentage of all the bars in the stack and the line series becomes incompatible with it.
To display the series in your scenario, you can use couple approaches. One thing that you can try is to use the Stack mode instead and calculate the values of the bar series in your view model, so that they stack as they would in Stack100. In this case you will have to set the Maximum property of the linear (vertical) axis, in order to prevent it from leaving an offset between its max value and the max value plotted on the chart.
Another option is to define an additional linear axis as a static resource and use it for all series which have Stack100 as CombineMode. You can read a bit more about multiple axes in this documentation article
I hope this will be helpful. Please, let us know if you have any further questions.
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