Gets or sets the extent of the feature which is closest to the base (e.g. a bulb) with a value from -1 to 1. Note: Only some needle shapes respect this property, namely: NeedleWithBulb, RectangleWithBulb, TrapezoidWithBulb, and TriangleWithBulb. Values further from zero than 1 can be used to make this extend further than the normal radius of the gauge.
Target Platforms: Android 4.4+, iOS 8+
Development Environments: Visual Studio 2015+, Visual Studio for Mac Preview, Xamarin for Visual Studio 4.2+
