org-agenda-use-time-grid

org-agenda-use-time-grid is a variable defined in `org-agenda.el'.
Its value is

t

Documentation:
Non-nil means show a time grid in the agenda schedule.
A time grid is a set of lines for specific times (like every two hours between
8:00 and 20:00). The items scheduled for a day at specific times are
sorted in between these lines.
For details about when the grid will be shown, and what it will look like, see
the variable `org-agenda-time-grid'.

You can customize this variable.