I know this is an old question, but I tried my best to do it with a time period of more than 2 hours, so this can help someone. In addition, some of the answers do not actually plot against time, giving equal space regardless of duration.
First, as @jameswarren says, use a scatter graph. Then right-click on the horizontal axis and select Format Axis.
In the "Number" field, select "Time", and at this point you may find that your scale is a little crazy, because it chooses to scale the axis daily. So, go back to the Axis options and select Fixed for the minimum, maximum and basic units.
To set the unit to hours, enter 1/24 = 0.0416667 (I used half of this to get half-hour increments). To start this with a round number, multiply it by the preferred number of hours and enter it in the Minimum field. In my case 08:00 = 0.333333

Dazbert Mar 20 '15 at 12:46 2015-03-20 12:46
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