I'm generating sequence diagram with plantuml. It's all fine but lack of support to "Frame element", like notation in figure 1 at http://www.ibm.com/developerworks/rational/library/3101.html. Since your tool supports things like opt, alt ..., Frame element may be not easy to implement for you. Could you add it?
Another issue is that text in sequence box is not default to be underlined. Could you set it to default or add a global option to set it?
And I would also thank you for creating such an amazing tool! Even visio is available for me, I prefer to your tool!