Hello Dominik,
It is because you have defined vizType="dual_line" but only offers 1 measure value. To plot a dual_line chart, you need to have, at least, 2 measure values and assign 1 to valueAxis and 1 to valueAxis2 like below:
<viz.feeds:FeedItemuid="valueAxis"type="Measure"values="Actual"/>
<viz.feeds:FeedItemuid="valueAxis2"type="Measure"values="anotherActual"/>
Of course, you need to define this "anotherActual" as Measure under flattenedDataset beforehand.
Hope this helps.
BR,
Chapman