I personally don't think that there is any advantage trying to free-draw a spiral line on a face-on view. If I did, I think that it would approximate the overall angle of that yellow line.
The following capture photo taken from a birds-eye view swing video may help you make a point.
I have placed a red X over the position of L5 and C7 vertebra (based on my best guesstimation as a person with a BSc degree in anatomy and a MD degree).
I have not drawn connecting lines because I think that the spine spirals too much (especially in the mid-upper thoracic spine region). However, it cannot be too far from an imaginary straight line drawn between those two points.
I am sure that you have stood behind a golfer at the end-backswing position and run your finger up his spine to get an idea of how the spine spirals.
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