I ran the following script on my Centos 5.2 VM and all I had to do after was add the crontab entry manually and restart mrtg. I got pretty pictures like these:

NOTE: You must first install gcc and gd-devel or the script won't work. If you already have anything under /var/www/html/mrtg delete all of it. Make sure httpd is installed and started.

#!/bin/bash

echo "MRTG Installer 1.0 ---------------------"
echo "By: Matt Savona (www.serveio.com)"
echo -e "----------------------------------------\n\n"

USER=`whoami`

if [ "$USER" != "root" ]; then
        echo "You must be logged in as root to use this installer!"
        exit 0;
fi

echo "Checking for previous MRTG installation..."

if [ -d /var/www/html/mrtg ]; then
        FILECOUNT=`ls /var/www/html/mrtg | wc -l`
        if [ "$FILECOUNT" -ge "1" ]; then
                echo "There appears to be files in /var/www/html/mrtg already! Exiting..."
                exit 0;
        else
                echo "The directory /var/www/html/mrtg exists, but there are no files. This is okay..."
        fi
else
        echo "The directory /var/www/html/mrtg does not exist. I will create it..."
        mkdir /var/www/html/mrtg
fi

echo "Creating necessary directories and preparing for installation..."

mkdir /var/www/html/mrtg/core
chmod 700 /var/www/html/mrtg/core

WD=`pwd`
cd /usr/local/src
rm -rf mrtg*

echo "Preparations complete..."

echo "Downloading the most recent MRTG and installing it..."
#wget http://people.ee.ethz.ch/~oetiker/webtools/mrtg/pub/mrtg.tar.gz
wget http://oss.oetiker.ch/mrtg/pub/mrtg.tar.gz
tar zfx mrtg.tar.gz
cd mrtg*
./configure --with-gd=/usr/local/lib
make
make install

echo "MRTG has been installed..."
echo "Updating MRTG config and template..."

wget http://www.serveio.com/da/mrtgcfg.tar.gz
tar zfx mrtgcfg.tar.gz
chmod +x system
mv -f system /var/www/html/mrtg/core
mv -f mrtg.cfg /var/www/html/mrtg/core

echo "Update complete!"

echo "Adding MRTG to /etc/crontab..."
echo "*/5 * * * * root /usr/local/mrtg-2/bin/mrtg /var/www/html/mrtg/core/mrtg.cfg" >> /etc/crontab
echo "" >> /etc/crontab

echo "Executing MRTG for the first time (you may see some warnings)..."
/usr/local/mrtg-2/bin/mrtg /var/www/html/mrtg/core/mrtg.cfg
echo "If you see warnings above, it is generally safe to ignore."

echo "Cleaning up installation..."
rm -rf mrtg*
cd $WD
echo "Installation of MRTG complete!"