onset of oxidation for cyclic voltammetry I have plotted a cyclic voltagram for my different polymer blends and I am trying to work out the onset of oxidation. picture below is my graph

from this website: https://www.ossila.com/pages/cyclic-voltammetry
There is a diagram which shows how the straight line should be placed so that it intersects the horizontal line, and thus the onset of oxidation can be found

However I am being told to draw my lines like this:

The procedure I am being told to do, doesnt make sense to me, I dont think the data from that would be useful. I would only get values for the Eonset that are very similar if I were to do it the way I'm being told. From my data, theres a clear difference in oxidation slope, so wouldn't it make sense to plot the diagonal line against that slope, rather than a small section near the beginning? From my 5 cyclic voltagrams, it would make much more sense if I were to do it as the website has told me to. There are more pictures on google which do it that way too
Please don't apply the reversible CV diagram on something which is clearly a non-classical cyclic voltammogram. When you experimentally don't get the shape as predicted by the theory, it means it that particular theoretical model does not apply in your case.
Look at three different types of voltammograms in this picture. Does your CV look like a reversible one? If not, then you were comparing an apple with an orange.

There is a very nice article which is relevant to you case:
Practical Aspects of Cyclic Voltammetry: How to Estimate Reduction Potentials When Irreversibility Prevails,  Journal of The Electrochemical Society, 2019,  166(5):H3175-H3187
This figure is relevant:

Examples of cyclic voltammograms showing chemically reversible and irreversible oxidation with designated potentials: E a = anodic potential, E c = cathodic potential, E (i) = inflection-point potential, E (p/2) = half-peak potential, E (e) = edge potential.