The graph looks more like a plot of temperature rather than VWC. Are you sure you’re plotting the right column of data? What are you using to actually record the data? Your sketch doesn’t output the data to a file or even print it in a easily-captured string out to the serial port, so I’m wondering how you collected multiple days of data. Plot your other variable and see what it looks like. The code also shows that you’re using 2 sensors. What does the data from the second sensor look like? The easiest way to see if a probe is working properly is to install 2 of them in the same location and in the same media and see if you get similar results for both of them.