before, some properties, such as colour could take on the following
values:

   color: <literal_def> | <series_def>;

where

   literal_def := <literal colour>
   series_def := var(<variable name>) | csv(<file>, <column>)

the new scheme is to separate such properties into two properties
each. for example:

  color: <literal_def>
  colors: <series_def>