What I use to capture data and create simple graphs for my site, this runs every minute via cron.

neidetcher_access_log.sh

#!/bin/sh

name=neidetcher_access
rrdfile="/var/local/${name}.rrd"
imageDir="/home/demian/code/demian0311.github.com/_site"

case "$1" in
	create)
		rrdtool create $rrdfile --step 60 \
			"DS:${name}:GAUGE:120:0:1000000" \
			"RRA:AVERAGE:0:1:60" \
			"RRA:AVERAGE:0:60:24"
	;;
	update)
		http_value=`/usr/sbin/logtail -f /var/log/nginx/neidetcher.access.log \
			-o /var/log/nginx/.neidetcher.access.log.offset | wc -l`
		https_value=`/usr/sbin/logtail -f /var/log/nginx/ssl_neidetcher.access.log \
			-o /var/log/nginx/.ssl_neidetcher.access.log.offset| wc -l`
		value=$(($http_value + $https_value))

		rrdtool update $rrdfile N:${value}
	;;
	dump)
		rrdtool dump $rrdfile
	;;
	graph)
		rrdtool graph ./${name}_hour.png \
		        --start end-1h \
		        DEF:${name}=${rrdfile}:${name}:AVERAGE \
		        AREA:${name}#aa5555:${name}
		mv -f ./${name}_hour.png ${imageDir}/${name}_hour.png

		rrdtool graph ./${name}_day.png \
		        --start end-24h \
		        DEF:${name}=${rrdfile}:${name}:AVERAGE \
		        AREA:${name}#aa5555:${name}
		mv -f ./${name}_day.png ${imageDir}/${name}_day.png
	;;
	*)
		echo "usage: $0 {create|update|graph}"
	;;
esac