Hello Nicolas,
I'm assuming that you are probably running a 9.4.x version on your 6900. FWIW, this graph doesn't exist at all in v10. I have a 9.4.5 box (single tmm) and a 10.0 box (running cmp). I see the graph in 9.4.5 but don't see it at all the 10.0 box. Since I only have one tmm in my 9.4.5 box, I can't see the problem you're talking about on my box.
There's a new dashboard in 10.0 though.
If all your virtuals are showing "mode: all" then I'd expect to see lines in all TMM's. So this means one of two things:
1. Either the graphs are not showing the right thing - very likely. So you'll have to work with support so they can show you in a different way that all tmm's are handling traffic (e.g. by using some CLI options)
OR
2. Your virtuals have been demoted to "single" mode (so "b virtual show all" is showing the wrong thing) and only tmm1.0 is handling traffic. Then you'll have to work with support and/or figure it out yourself by reading docs and looking at your configs to see why your virtuals might have got demoted and then work backwards to get them to "CMP mode all" again.
I know if you use global variables in irules then those virtuals get demoted.
Hope this helps.