This addon is quite simple.

Since 1.10 you can watch a Faction, that means you select a Faction and its reputation progress is shown as an experience bar.

This mod sets the watched faction to the faction you last gained reputation from.

If you gain Reputation from the Cenarian Circle the reputation xp bar will switch to the Cenarian Circle.

It updates every time you gain reputation from a different Faction.

This is especially helpful while farming reputation from many Factions, like the bg factions etc.

Factions that are set to inactive will not be handled.

Use /drb toggle or /dynrepbar toggle to activate/deactivate the mod

Thos mod does not implement an own bar. It controls the blizzard reputation bar or other reputation bars (when available)

Patch notes for Dynamic Reputation Bar
25.5.2007
-updated

7.2.2007
-Ace
-/dynrepbar toggle and /drb toggle will deactivate/activate the mod
-ignores Inactive Factions

No Bug with this addon, but:
May not work with addons that come with an own bar, that replaces the Blizzard bar. (They may not notice the automatic change)