How to set a constant speed along one axis of an object following curve
 

I need to borrow someone's brain please!

Short version of the question:

How can you set a constant speed along one axis of an object following curve?

Longer version of the question:

Im trying to make heart monitor ECG / ECG graph.

Those monitors where a dot travels from left to right and draws the heart rhythm.

So I created the curve in Maya by tracing an existing image. That went well.

I created a small sphere to represent the dot and attached it to the curve as a motion path. That went well too.

The problem I have is that in real heart monitors the dots travels from left to right at a constant speed. It does not slow down as it draws a spike.

When travelling along the Z axis, my dot travels at a constant speed along the curve, but not a constant speed along Z, so it slows as it draws the spikes. So it looks wrong.

I've tried baking the simulation and going into the Graph Editor and equalise the the motion along Translate Z, but then the sphere no longer follows the original graph / curve.

How can I get the sphere to follow the motion path perfectly, but maintain a constant speed along the Z axis?

I uploaded this animated GIF but it has been converted to a static JPEG for some reason! :help: