Nawqaan 232

Reflecting users initial and subsequent constituent selection in the MRB load-at-a-glance graph popup. Basic error handling implemented too. Data retrieval and actual plotting pending.
I changed the constructor invocation, but I left the constructor itself the same. In this case I think it's ok to leave these properties with value 'undefined' -- it has the same effect in the rest of the code that defaulting to a 'null' value would accomplish.