Specifies how the angle (in degrees) is calculated from the feature attributes, using an Arcade expression. The calculation may use attributes, fixed numbers, or a combination of both. The default value is an ArcadeExpression containing an empty expression string, which will produce a zero angle when evaluated. Warning: If the expression attempts to use attributes which don't exist then no label will be produced. This is distinct from the attribute existing but having a null or empty value, which will be interpreted as a zero angle.