Generally speaking, Nagios is used for alerting based on some present condition, and something like cacti is used for recording values and storing them for historical analysis. Nagios can do some basic trending graphs, but nothing compared to cacti. When you say you want to be able to 'chart', that implies advanced historical graphs / analysis, and that means cacti. (Or other similar solution of your choice.)
That being said, I personally would choose an approach that is not necessarily the simplest, but could be the most useful going forward. It is a two-step process. The first step is to devise a method to extract the value from the log file. The second step is to get that value into a tracking system.
For the first step, I would suggest setting up net-snmp on the server where the log file is stored. Write a script, or find one that's been written already, that can extract the value from the log file exactly as you'd like to store it. You can then customize snmpd to run this script for you, and return the value with a custom OID that you specify.
For the second phase, you'll direct your graphing tool (cacti) to contact the server in question, and query for the specific OID, which in turn invokes the script and returns a value. This will get plotted into your historical graphing solution.
If you're talking monitoring the current state of the latest entry in the log file, you're talking Nagios. Write your own script, as above, but this script becomes a Nagios plugin that is invoked by say, nrpe.
This snmp-based approach is a bit of up-front work, but can yield great value down the road as you can begin tracking all sorts of custom values in your environment and getting them graphed.
Hope this helps.