SignalXY: Fill above/below does not render

the property "FillType" of SingalXY does not work.

this code is writing in winform:
```csharp
double[] data = DataGen.RandomWalk(1000);
double[] xs = new double[1000];
for (int i = 0; i < 1000; i++)
    xs[i] = i;

var sig = formsPlot1.Plot.AddSignalXY(xs, data);
sig.FillColor1 = Color.Blue;
sig.GradientFillColor1 = Color.Transparent;
sig.FillType = FillType.FillBelow;
```