This was reported to the Debian BTS.
http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=232685

"The network graph scaling doesn't work very well; I want it
to typically show my DSL bandwidth but it inevitably scales to 
the local LAN bandwidth after time.  Two ways this might be fixed:

1) Add an option in context menu to reset autoscaling,
   so it will re-fit to the lower bandwidth on demand
2) Option in prefs to set fixed scaling

With #2, being able to add two network applets on the same interface
(one with DSL scaling, one with LAN) would be cool too.

The "obvious" way to do #1 by deactivating the network graph and
reactivating it doesn't actually work, because the static variables in
linux-proc.c actually cause it to break badly (all data during the
"off" time is shown in one big spike when it restarts, messing up
scaling)"

and

"The default behavior actually seems much worse in 2.14.3-2 (I had been running the 2.10 binary, so I hadn't noticed).  The scaling now makes no sense at
all -- it seems to change dynamically and unpredictably.  For example:

- Restart the applet
- scp transfer at 3MB/sec -- shows about 40% usage
- wget at 8MB/sec -- shows about 100% usage
- scp transfer at 3MB/sec -- doesn't show anything
- wait 5 minutes, then try the SCP, it's back to 40%

I think being able to set a fixed scaling (e.g., specify in the
preferences that a full graph should correspond to 100Mbps) is the
only behavior that would make sense... autoscaling without any
indication of the chosen scale makes a graph useless."