cron

#!/bin/bash
#Run this script via a crontab entry whenever you want your drive usage sampled
FILE=/Library/size_history.csv
MOUNTPOINT="Library"

echo "$(/bin/date +%Y/%m/%d),$(df -B G | grep "${MOUNTPOINT}" | awk '{print $3,$2}' | sed -e 's/G//g' -e 's/ /,/')" >> ${FILE}