When setting or getting the Norm attribute, the attribute name must be qualified by an axis index in the range 1 to 3. The 3 elements of the Norm attribute are together interpreted as a vector in 3D graphics coordinates that is normal to the plane upon which text and marks should be drawn. When testing or clearing the attribute, the axis index is optional. If no index is supplied, then clearing the Norm attribute will clear all three elements, and testing the Norm attribute will return a non-zero value if any of the three elements are set.
The default value is 1.0 for each of the 3 elements. The length of the vector is insignificant, but an error will be reported when attempting to draw text or markers if the vector has zero length.
AST A Library for Handling World Coordinate Systems in Astronomy