I'm getting weird numbers out, in the range of 13-2, at least that's what is showing on brewpiless (the gravity scale does't seem to relate to the numbers on the graph either). I figured I'd come back to it as I didn't want to mess around with a brew but as time went on I realised the actual numbers (and accuracy) aren't important - the curve is what matters. Once it flattens out I take a couple of hydrometer readings and they confirm what the curve is telling me.
In this image note the dip in the green line where I preprogrammed a cold crash, the curve (purple line) clearly showed me that fermentation hadn't finished. I learned not to programme that in advance.
What is your calibration data?
Did BrewPiLess calculate the gravity, or iSpindle?