The StepLine series type is similar to the LineSeries one, but it does not use the shortest distance to connect two data points. As the LilneSeries requires at least two datapoints in order to draw a connecting line, this requirement is also valid for the StepLine series. I would suggest you to use BarSeries instead, for the case when you only have one datapoint.
I also noted that you have opened a support ticket with the same question. Please, keep the conversation in one thread so it is easier for both sides to follow.
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