If an AxisAxis has an active Unit attribute, changing its value (either by
setting a new value or by clearing it so that the default value is
re-instated) may cause the Label and Symbol attributes to be changed
accordingly. For instance, if an Axis has Unit, Label and Symbol of ``Hz'',
``Frequency'' and ``nu'', then changing its Unit attribute to ``log(Hz)''
will cause AST to change its Label and Symbol to ``log(Frequency)'' and
``Log(nu)''. These changes are only made if the Unit attribute is active,
and a MappingMapping can be found from the old units to the new units. On the other
hand, changing the Unit from ``Hz'' to ``MHz'' would not cause any change
to the Label or Symbol attributes.