artifactId should be websequencediagrams-maven-plugin

[bohni]

see http://maven.apache.org/guides/plugin/guide-java-plugin-development.html

Important Notice: Plugin Naming Convention and Apache Maven Trademark

You will typically name your plugin <yourplugin>-maven-plugin.

Calling it maven-<yourplugin>-plugin (note "Maven" is at the beginning of the plugin name) is strongly discouraged since it's a reserved naming pattern for official Apache Maven plugins maintained by the Apache Maven team with groupId org.apache.maven.plugins. Using this naming pattern is an infringement of the Apache Maven Trademark.