I think there are 2 issues with the graph.
1. Your example doesn't graph anything because you are giving it no data 🙂
2. You are not including the day, so if you look at your math on the time, you are giving the graph the exact same values each day as it rolls over. You may be able to incorporate a day addition into the math and get it to work.
However take a look at my example. I used absolute time, but just hid the day and years etc. When you configure the grpah properties, select advnaced settings and use this time string %<%H:%M>T
Just run the example and then change the time in the timestamp to the next day and you can see how it behaves.