Can I confirm I have this right ...

If I want to have a DS showing if the measured value was above some threshold 
(in this case, network traffic above say 80%) I can use a compute DS like :

So if the "in" value is greater than the threshold value (nnnn here) then pc80 
will be 1, and this will get consolidated so that later I'll be able to see 
(for example) a consolidated value that might show (say) 0.2 meaning that the 
"in" value was above the threshold for 20% of the time during that consolidated 

And I can add this with rrdtune with
  rrdtool tune somefile.rrd DS:pc80:COMPUTE:in,nnnn,GT

What about if (or rather, when) I need to change the thresholds later ? 
Specifically I'm thinking that if we currently have (say) 100Mbps service, and 
we upgrade it to (say) 200Mbps, I'll need to change the threshold (and thus the 
computation for new data) without affecting historical data. I don't see an 
option for rrdtune to do this.

Lastly, in what version does rrdtune get to add a DS ? I have 1.4.7 (Debian 
Wheezy) and the man page doesn't mention adding or deleting DSs.
On this, could I suggest that when documenting (the online docs are great BTW) 
new features, adding a small "added in version x.y.z" would be helpful given 
how much development there's been.

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