If no value has been set to an attribute, the value of the
attribute in the generation of the chart will be the default value of
the attribute given in the XML reference. One step above default values
are the values of the attributes given in a plot
tag. Therefore, the attributes given in a plot tag can be
considered as user defined default values.
The plot tag has also a few plot specific
attributes. These attributes do not differ from other attributes in
plot tag in any other way but they can not be overridden
by attributes given in other tags.
