Plot Trendline in signal processing

Hi I'm calculating muscular activation and to determinate fatigue, I also get the median value, but I'd like to plot a trendline to that median value. ¿How can I do?, and if possible hpw to calculate max and min value Thanks

[José-Luis]

What's a trendline to that median value mean? Do you want to force the line to pass through that value? What are your two variables?

[Julio Martín]

Hi I mean how to show a trendline in a figure, when I plot data Thanks