If a model is validated at:
http://www.sbsi.ed.ac.uk/html/sedml/index.html
and that models contains a plot, it is marked as invalid:
cvc-complex-type.2.4.a: Invalid content was found starting with element 'plot2D'. One of '{"http://sed-ml.org/":report}' is expected. at line 56
The online validator no longer exists, it is now at tools.jlibsedml.org, and this problem is not seen.