In addition to the plot of my metrics, i want to display the trend of the metrics.
E.g. the HRV is going down over the defined date range.
With the help of excel its really easy to generate and plot this trend line within a diagram.
As you can see in my attached screenshot, I can clearly see that the HRV trend is going down. With this knowledge, I can push or slow down my athletes.

In my opinion, a linear interpolation of the values ​​of the selected range should be enough.