"Math"
You could easily employ a series of graphical assets and basic mathematics to plot graphics along a curve.
Worst case you would just brute force your way through it.
Just as well, it's usually inefficient to draw everything you need, using pre-rendered assets is generally more efficient and preferred. Even in systems which provide drawing primitives/APIs the API calls are little more than instructions that are interpreted by a graphics engine (be it hardware or software.)
Regardless, here are some interesting things to note:
- IceHud weighs in at 1 MB, that's tell-tale that it ships with a fair amount of graphical assets.
- IceHud unpacked contains 50 different BLPs.
- Of the 50 BLPs present, a quarter of them appear to represent the Bars you see in IceHud screenshots.
My suggestion is that the next time you see an AddOn that does something you wish you could do you should first download it then inspect the contents.. not only should it be a learning experience, you may actually save yourself some time and trouble.