When the program starts, pick a diagram type from the File->New menu.
Once you are confident that you like Violet, you should download it and run it locally. Unfortunately, WebStart ignores the suggested default extensions.
However, the sandbox imposes a few restrictions. Note: The Web Start version is functional and runs safely in the “sandbox”-it can't do anything on your local machine without you agreeing to it. If your browser doesn't support Web Start or the planets aren't aligned, run javaws If Java Web Start is properly configured in your browser, and all planets are aligned properly, clicking launches Violet. If not, you need to associate the MIME type application/x-java-jnlp-file with the javaws program in the bin directory of your Java installation. The Java installer should have done this. Java Web Start needs to be properly configured for your browser.
If you don't have it, download and install the JDK or the JRE. With Java Web Start, you can try out Violet without installing it.
The UML software should allow you to create all types of diagrams.