Further information. After several failed attempt to redirect the output of serial_mon.py to a file, I was finally able to see an event in real time in terminal while I was sitting there. the Temp certainly didn't go from 17C to -40C nor did the Press and Humidity change.
So it doesn't seem to be a server/graphing issue, rather looks like the sensor I purchased maybe bad? (I'd still like to know if it's possible to graph slightly more often than 1/hr, but totally understand if it's to discourage overloading your server).