Layer is an element that contains different objects (e.g. images) to define the visual presentation of a gauge and its elements.
Layers can be added at two levels:
Every new layer is painted behind all other gauge elements, because its GaugeOptionsBase.ZIndex property is set to -100 by default. If you want to bring this layer in front of other gauge elements (or paint it above other layers, if there are any), you need to set the ZIndex property to a greater value.