How to add trendline

I have a graph like below.

How can I add a trendline line to it, like a trendline black stripe in the below figure?

Answer 1

[Voss]

Look into movmean.

Answer 2

[Star Strider]

There are any number of approaches, including lowpass filters of various descriptions.

Another one would be polyfit and polyval, perhaps with a degree 4 polynomial.

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