Gaps in hourly graph

The values are as expected but I just can’t seem to get rid of these nasty gaps.

name: interface
time                      first      last       last_first
----                      -----      ----       ----------
2018-10-28T10:00:00+01:00 2002210907 2002340137 0.12922999999999998
2018-10-28T11:00:00+01:00 2002400592 2005548691 3.148099
2018-10-28T12:00:00+01:00 2005562631 2356795716 351.233085
2018-10-28T13:00:00+01:00 2388283572 3589820775 1201.537203