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 👍 4hoursinterval-bad has ~ 300 vertical ...

This MEP would add a mechanism for creating a grid of subplots based on the number of axes to be plotted and a strategy for how they should be arranged, with some sensible strategy as the default.