SignalXY: add support for markers

**Question:** Why are marker missing in version 5 for SignalXY plot while in version 4 there is?

**ScottPlot Version:** 5.0.23

**Code Sample:** 

Version 4
```cs

var sig = plt.AddSignalXY(xs, ys);
sig.MarkerShape = MarkerShape.filledTriangleUp;
sig.MarkerSize = 10;

```