ScatterPlot: Add OffsetX and OffsetY

**Feature description:** Scatter plots should have X and Y offsets to allow "shifting" display of the data without requiring modification of the values in the arrays

**Code example:**

```cs
var sp = plt.AddScatter(xs, ys);
sp.OffsetX = 123;
sp.OffsetY = 456;
```

This idea was raised in #1002 and implemented in #1111 (not yet merged)