Check stability of scatter plots with a single point while zooming

@saklanmazozgur commented in #764:

```cs
double[] xs = new double[1];
double[] ys = new double[1];
Scatter = Angle_Angle1_Graphic.plt.PlotScatter(xs, ys);
```

If the graph is then zoomed, it gives the error "The parameter is not valid". 

It does not give this error when the size of the array is more than 1.