Hello, I’m studying IS Analysis, and Software Engineering has UML, we were doing some State Diagrams, and I found your software (online) does not have junctions nor concurrency bar (black thick bar that receives the satate’s arrows, a la Activity Diagram’s “Parallel” example. It may be just representation, but the lack of these two items forbids me to use your application for this. Also, sequence diagram is quite limited (compared to excellent http://www.websequencediagrams.com).
Regarding the sequence diagrams limitation. This website uses the PlantUML engine directly, so it will only do what you would expect from that software at http://www.plantuml.com. We have no control over that. Take a look at the PlantUML website and see if they have a solution.