I am not sure if I have the same issue. If you see this, please let me know.
I sorted my data using day of week as a custom list for the sort. I used that data to create a chart and would like to add a major gridline between each day of the week. The number of entries per day is variable, so I cannot add a major gridline every n days.
I spoke with Microsoft support. They escalated the question and I am waiting for a response.