add an ASCII illustration of 60-min values in data-aggregation

[williamhobbs]

An ASCII illustration for the 60 min interval aggregation would be helpful.

The 30-min section includes:

Timestamps: 00 05 10 15 20 25 30 35 40 45 50 55 00 05 10 15 20 25
Interval 1:          --------------------
Interval 2:                            --------------------

It would be nice to add this to the 60-min section:

Timestamps:  40 45 50 55 00 05 10 15 20 25 30 35 40 45 50 55 00 05 10 15 20 25
Interval 1:     --------------------
Interval 2:                                         --------------------

Edit: now I think I got it wrong, and it should be:

Timestamps: 10 15 20 25 30 35 40 45 50 55 00 05 10 15 20 25 30 35 40 45 50
Interval 1:    --------------------
Interval 2:                                        --------------------

[kandersolar]

Addressed by #18