Successfully replicated your solution and it removes all mentioned problems.
But discrete timestamp is not good solution for my use case because this solution is going to be used on dashboard with many sheets and timestamp with one year period. And that would be a lot of scrolling with discrete timestamp.
Continuous timestamp results in view with all year data and no need for scrolling.
After a short try out, we'll most probably go with the approach with "discrete timestamps" like you suggested, but since the original questions is not answered I would mark this thread as "this helped me", and not "correct answer".
In our case, using continuous 'Date & Time' type filed in a graph results in a quirky rendering/handling of daylight saving time. It would still be good if someone can explain it.
Thanks again for the workaround.