ARIA Graphics Module -- proposed roles hierarchy & data properties

Hello all,

I finally had time to write down some ideas about the more complex graphics
role model Fred has developed.  Rather than trying to directly edit the
full spec text, I've created a summary document (attached as HTML).

For the roles, I've organized them as a hierarchy of classes: first the
different types of documents, then the different types of objects.  You'll
notice that I've left out many of the roles from Fred's taxonomy.  However,
if you read the descriptions, I think all the different functions are
covered.  As I've mentioned in the teleconferences, I think we need to
focus on the generalizable data semantics rather than getting bogged down
in particular representations or names.  E.g., map graticules, chart
gridlines, and tickmarks all serve the same function.

For the properties, I've outlined the basic structure for assigning data
values to data points.  I haven't yet figured out how to represent
connectors and network graphs.

There are a number of outstanding Issues I've noted; you will surely come
up with others.  If you have major questions, I can hopefully clear things
up on Friday's call.  Otherwise, we could probably put it on the agenda for
next week.

Best,
Amelia