Hi Niels,
Great observation, and you are correct that you can identify the potential in that manner. If you had a nice digitizer and clean data set, then your method might be easier. If, however, you collected the data with an XY Recorder (https://davidpace.com/old-school-the-xy-recorder/) or had noisy data, then the graphical method is pretty good. Derivative calculations are prone to high variance unless you apply pre-processing such as smoothing or other techniques. The logical choice is whichever method works for your particular case.
Thank you for your feedback!