It appears that there is a limit (quite low ~ 500) of vertical grid lines that can be represented in a matplotlib graph.

I attached 3 graphs with minor grids 👍

1. 4hoursinterval-bad has ~ 300 vertical grid lines, but should have completed the whole X axis would have ended up with 600 vertical grid lines.
2. 4hoursinterval-good has ~ 420 vertical grid lines (reduced time interval)
3. 8hoursinterval-good has ~ 300 vertical grid lines. It covers the whole required time interval, but at a lower resolution.

Therefore, I assume that there is either a bug somewhere or there is a limit in the code. If there is a limit is that documented anywhere?