Bug: graphs with 'GROUP BY "smth"' show not all values of smth

Suppose I have some metric with `smth` tag.

If I create a graph for this metric with `GROUP BY "smth"` the resulting graph should have a line for each value of `smth` (assuming there is data for it).

I stumbled upon a case where on 15 minutes up to 7 days time span everything is ok, but on longer spans like 30d there are graphs only for a fraction of values.

For example, if there are total of 12 values for `smth` tag, I can see 12 graphs on 1d-7d span, but only 4 graphs on 30d span. However, if I remove `GROUP BY` and select all 12 values by hand then all 12 graphs are shown.

Sorry for the lack of screenshots/MWE - the data is private and I was unable to reproduce this bug with fake data.

Chronograf v 1.7.14, Debian 9 (pre-built package).

No files, tests, or code entry points are named. Start by reproducing the GROUP BY behavior across the reported time spans in Chronograf 1.7.14 with comparable tag data; done means every available smth value produces a graph on longer spans as it does on shorter ones.

Written by the indexing model from the issue text.