It has always been a wish that I would find a cross platform alternative program that can do at least as much as MS Viso. I think that I found one. LucidChart.com. While it is cloud based (with an "offline" version via Chrome) it does meet and exceed all that I need to do when I need to draw MS Viso diagrams. While the "free" basic version will get you started you will find that having the premium version will unlock much more potential like importing your own stencils. In fact, I tend to go back and forth on the premium version and the basic version depending on the work that I am doing and the length of contract that I am working on.
So next time you are looking for a MS Viso like alternative, take a look at LucidChart.com. I think you will find value like I have.