Soil Humidity sensor value rising over time
The datasheet says when the probe gets drier, the analog output voltage goes up, and conversely goes lower when wet.  Is your graph scale upside down, or you need to divide into 1 somewhere?  I think maybe your logic says graph go higher when voltage goes up, but for a Y scale that is wetter at top drier at bottom, this logic is inverted.
What measures do you get when you dry it off and hold it in air, versus putting it in a glass of tap water?