There is a VISIO alternative
www.lucidchart.com. It works quite well considering it is a web based app. I was impressed whrn I tried it.
DIA isn't bad but it has a few quirks.
If you google for "visio alternative multiplatform on-line free" you'll get a number of hits. But most reviews are not that useful. Often OODraw is recommended, but I can't take those drawing capabilities seriously.
Based on that reviewing quality I am afraid you have to try out some to see what fits your needs.
PS: I know you did not ask for a Visio alternative. But as it happens Visio is quite good at what you do.
jlinkels