Some bosses have special mechanics that place an additional UI bar on your screen. This addon gives you a very plain looking bar in addition to that, that displays the same thing, but being more plain it's in some cases also much more clear. Also, this bar comes along with a numeric display, which the original bars don't, allowing you to accurately gauge how much of the bar is filled rather than having to estimate. Finally, this bar can be moved.
Normally, the bar will appear once you start a fight that uses such a bar, and disappear when you leave combat. You can 'force' it to appear using the slash command /altbar, which is also the command to hide it if it's already shown.
If you want to hide the bored and/or background of the frame, use "/altbar border <on/off>" and "/altbar background <on/off>".
Note that this addon will need the original bar to be shown in order to function. It does not have to be placed anywhere you can actually see it, but it needs to be 'visible' to the UI.
Also note that this addon has only been tested on Onyxia's lightning charge, as the other two bosses with bars are down for me this week. It should, however, work just the same on both
1.2: Added functionality for hiding the border and background of the frame. Fixed a bug that would sometimes reduce the bar to 0 on Cho'gall until the next display update (untested, but should work - update next week if not). Found a better (and hopefully more future-proof) way to hook into the display function.