How to make a progress-speed bar more understandable?

I've just installed Windows 8 and noticed the brand new File copying dialog. When expanded, it shows the progress and the current transfer speed in one graph. However, the graph confuses me greatly. The faster it copies, the quicker progress increases (left-to-right) and the higher the graph (bottom-to-top). I think this makes the speed graph non-linear, and I have trouble reading the graph. This makes the graph no more than a toy.

^{It's in Dutch but I think the gist is clear.}

I assume Microsoft's design team have discussed all pros and cons of this graph, so there must be some merit to this design. I would like to write a custom control for my own application that also shows progress and speed in one bar, but I don't think this is the way to go. How to make the speed part of the bar more understandable and useful? What are the alternatives?

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To illustrate the problem I have with it:

This graph indicates that for half of the total transfer time, the file was copied at half the speed (e.g. 30 seconds of half speed if the total copying took one minute). That would be useful information but is not clear to me from the graph.