This paper explores how to incorporate the expressiveness and learning benefits of diagrams with the computing power and scalability available in online learning. As a first step toward a framework for automated assessment of student diagrams in online learning environments, we propose a new feature for the INFACT system?s Sketch Tool. This "Blobs, Links and Labels" (BLL) feature eliminates ambiguity in diagrams by imposing a flexible, graph-inspired structure to the diagrams. Specifying assessment will be done in the style of programming-by- demonstration, where the instructor uses the system to provide examples of correct or incorrect student diagrams, which are then used by the assessment tool. In designing the BLL feature, we aim to strike a balance between diagrams? expressive power versus ease of use and ease of automated assessment.