Under the title Time grid in the Global Options dialog box, you specify the desired density for gridlines scaling a time line reference axis. In order to use this option, the Time grid option under Grid type must be turned on.
If you choose the Fine value for the time line, one gridline is drawn for each time unit interval. Thus, if the time unit being used is Days, one gridline is drawn for each day.
If you choose the Coarse value for the time line, gridlines are drawn for the more inclusive time unit used on the top half of the time line. Thus, if the active time unit is Days, days are displayed on the bottom half of the time line, and months on the top half. In this case, a gridline is drawn for each month.
The Time grid option is available only in the Global Options dialog window that appears for reference axis graphs with time line scaling.