This is a plugin for Track-O-Matic which allows you to track your character's free bag space.
Works with Track-O-Matic version: 1.6.0+
To add this plugin's bar to the tracker, click the magnifying glass icon at the top of the tracker to open the main menu and select the plugin from the "Track Plugin" submenu.
When added, the bar will appear under the "Misc. Info" category header.
By default, this plugin will ignore profession bags (enchanting bags, herb bags, etc.), but has an option to include such bags in the total. (right click the bar and select "Include profession bags")