I've got a question in two parts. From looking at some delta data, there seems to be some discrepancy with rollups. This may be a misunderstanding on my part to how the deltas/rollups are calculated. I'm also getting no graph for 15m intervals.

### Part 1

Sample data for a power usage accumulator:

```
timestamp value
21/8/2019 05:30:00 2724.95 kW·h
21/8/2019 05:15:00 2722.89 kW·h
21/8/2019 05:00:00 2720.82 kW·h
21/8/2019 04:45:00 2718.79 kW·h
21/8/2019 04:30:00 2717.55 kW·h
21/8/2019 04:15:00 2716.30 kW·h
21/8/2019 04:00:00 2715.06 kW·h
21/8/2019 03:45:00 2713.82 kW·h
21/8/2019 03:30:00 2712.57 kW·h
21/8/2019 03:15:00 2711.34 kW·h
21/8/2019 03:00:00 2710.09 kW·h
21/8/2019 02:45:00 2708.84 kW·h
21/8/2019 02:30:00 2707.59 kW·h
21/8/2019 02:15:00 2706.36 kW·h
21/8/2019 02:00:00 2704.24 kW·h
21/8/2019 01:45:00 2702.09 kW·h
21/8/2019 01:30:00 2699.94 kW·h
```

If we set the rollup interval to 15m (or 900 seconds) and rollup type to delta, the graph on the data point details page goes blank:

Bug or expected behaviour?

### Part 2:

For a 30 minute rollup and 60 minute rollup the graphs are as follows:

and the transcribed values are:

time | 30 mins | 60 mins |
---|---|---|

02:00 | 2.12 | 4.6 |

02:30 | 1.25 | |

03:00 | 1.24 | 3.73 |

03:30 | 1.25 | |

04:00 | 1.24 | 3.73 |

04:30 | 1.24 |

I would have assumed that the 1 hour delta is the assume as the combined 30 minute deltas but it seems not to be the case.