It would be great to be able to horizontally scroll through the recent past; alternatively, it would be great to be able to point speedometer at a "history" file to review prior traffic.
This may be "a bridge too far" on the mission creep scale but man, this would be a super useful front-line troubleshooting tool.
I know there are plenty of cacti and nagios options out there, but speedometer is such a great 'quick & dirty' view of traffic.. I find myself using it often. :-)