That would be great. I've opened an issue at https://github.com/PaxInstruments/t400-firmware/issues/199. I think adding another page to the graph button would be okay. There are some other possible features that could be implemented as alternate button functions after a mutton long press. This feature would fit into that system as well. See https://github.com/PaxInstruments/t400-firmware/issues/200
We currently have sixteen bytes available on the ATmega32U4, so we would need to move the lookup table over to the 1 MB SPI flash chip. Oh yeah! I've included an external flash chip for hacking just like this See https://github.com/PaxInstruments/t400-firmware/issues/191. In the mean time, this feature could be implemented as a configuration option that would only work if something else is disabled. @davidbuzz made the SD logging a configuration option http://community.paxinstruments.com/t/customising-the-firmware-to-do-other-things/138/2 , which saves about 8 KB of flash. If you can do without the logging, this would be perfect.
UPDATE: In commit 58df605a68b84739ac4c08a71941648d5f9a3cdd, setting SD_LOGGING_ENABLED to 0 will give a total 0f 8,606 bytes available.