JavaScript/Node.js is my preferred language. As far as I understood, it should be possible to do the calc based on the raw metric values provided with the sample data. But I think I'm doing something wrong. Thanks again for your effort!

Sorry, I'm still having problems with the calculation. Could you please give me an examle with the above provided sample data?

Sorry for being a total noob at this. You mean as described at en.wikibooks.org/wiki/Numerical_Methods/…

I already calculate the change rates. That's not the problem. Normally, the change rates (with my metric (except errors)) will be >= 0. What I want to with my graph is the "speed" that the values of the metric change from t0 to t1. If the "speed" slows down, I want to show a negative trend. This is IMHO not possible with just the change rate.

Have a look at codepen.io/anon/pen/PwxaRy Yet, the problem with the Y values remains... Do you have an idea?

A line gradient solution with D3 can be found at bl.ocks.org/d3noob/3e72cafd95e1834f599b and (better) codepen.io/ekrembk/pen/ights for example...

Thanks a lot for your answer! Maybe my question is not very clear, but I struggle more with the calculation of the Y value than with the diverging color scheme. I have the absolute values of the metric, but that doesn't really help me showing the "speed" of change, and if the "speed" is positive compared to the last "step"/"time bucket".