Your thinking is flawed for several reasons.

1. Why would you try to shoot someone in the kneecaps? In that situation, you want to shoot them in the torso/chest area.

2. If the robber has a gun, it's best not to threaten him with a knife or an axe. By the time you wind up to throw that thing, he'll have shot you 5 times.

3. If the robber has an edged weapon, your advantage won't come from being able to throw something at him, although it would help. Let's say you throw the axe and miss or just give him a nick on the arm? Now what? You'd need to carry around several throwing implements to make sure you stop him. Probably the best non-gun weapon in that situation is a sword. With a sword, you have a great advantage being able to attack him from 3-4 feet away, while his knife won't be able to reach you.

That being said, I think it's still not a bad idea to have something ready to throw at him. That axe looks a bit heavy and tough to throw at a reasonable speed. Cold Steel has some nice throwing weapons (and swords) that you might want to look at. I don't see them on the website anymore, but they had things called torpedos that were basically solid two-ended steel spikes.

Also, I have a video called "Making It Stick" by Michael Janich, published by Paladin Press. A great video if you want to learn how to throw knives, axes and spears well.