Piwik DB schema is interesting and we should try generate a pretty image of it. - http://piwik.org/docs/plugins/database-schema/
DBDesigner doesn't run on Ubuntu 10.04. What about the Windows version?
A good alternative to generating this schema is the Schema spy tool: http://qa.piwik.org:8080/schema/
I linked it from the doc page as well
from this page: http://piwik.org/docs/plugins/database-schema/
instead we will write guides such as: http://developer.piwik.org/guides