The agenda strategy allows you to quickly turn your day plan into a calendar-like timeline:

When this strategy is on you see a little calendar icon on the top right of your day plan:
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Click that to see your agenda for today. You can also use the key "L" to quickly switch between your day list and the agenda view.
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Any task that is scheduled for the day and has a time in the title will automatically be placed on the agenda. The length is determined by the duration estimate.

If you want to plan out your day in the agenda it is easiest to first make a regular day plan and then find a place for it in your day. To do that open the planning sidebar (X or clicking the date on the left), navigate to the Today tab and then dragging tasks in there. Make sure the tasks already have a duration estimate so you can see if they fit while dragging. 

You can also add a task directly by clicking anywhere into the agenda.

You can change the duration of an item by either resizing it or clicking into the title and overwriting the duration estimate.

You can easily zoom in and out with the "1" and "2" keys on your keyboard.

In the agenda settings (shift click agenda icon in day view) you can set your slot size. If you want items to snap to a grid every 5 min increment, drag the blue dot all the way to the left. 

To remove time from a task, shift-click it in order to be able to edit it, and then click on the "X" icon to remove the time.