With an AMD GPU, you will get the graph you are seeing, because they work differently. Only with an NVIDIA GPU, you will get the one more complex-looking graph that you see in the undervolting guides for NVIDIA.
You can still undervolt using Afterburner if you want, you just have to set it up correctly, like unlocking voltage control in the settings. And then, as explained in my first reply, for undervolting, you'd lower the right point of the bottom graph a bit, to lower the voltage under load. Then you'd check with Furmark and in games that the GPU is still stable, as well as monitoring things like power draw with GPU-Z, to see the effect of the undervolt.
But AMD Adrenalin also allows for simple undervolting, so might as well use that, should be under Performance -> Tuning in there. Note down the VCore you arrived at, and if a new driver version resets it, just re-enter it, it's a matter of 30 seconds to do this. Then you don't need Afterburner.