make a difference in graphs for data of "unfinished periods" and "complete periods" #10291

Graphs show data of of "unfinished periods" and "complete periods" in same way.*

Since imho comparison of data of "unfinished periods" with "complete periods" does not make big sense, there should be visiualization for the difference to trigger people to be aware of this problem
One idea is not connecting the data point of "unfinished period" to the others before even this period is finished.
This may look like this:

*why are there unfinished periods?
When comparing data e.g. number of visits, entry pages etc
with data of day before there is a problem:
when checking earlier than 23:59
there is always a difference because
the period taken into account is shorter
than the complete day before
and with this not all "things" of this day has already happened.

An example:
Looking at data at 9:00 in the morning:
actual data is taken from 0:00 to 9:00 = 9h
the data for comparison form 0:00 to 0:00 = 24h

This issue is an extract of
"comparison of data of "unfinished periods" with "complete periods" does not make big sense e.g. in widget" https://github.com/piwik/piwik/issues/10282
which addresses problem in general for data shown in table and in graphs

one other idea was to connect the data point of unfinished period with a dashed/interrupted line

but this approach would be much more "costly" to make it good looking and compatible at all places where graphs are used/shown...

so no connection at all should be fine :-)

another example:

a possible second step:
maybe one can show even an estimated forecast, e.g. like this?

@mattab would be nice to tackle this one at some point as it's always "irritating" for unfinished periods. Maybe there is a way to show the last dot and last line in a light red or something. On hover it could say the day is not yet finished but not sure how complicated it is to implement. Would be useful especially since we want to show more of such graphs.

This would be great, also at the top dashboard where the data of the unfinished day is compared to the completed day.

Added Major tag and moved into 3.0.0 milestone so we keep an eye on this important issue :+1:

see also https://github.com/matomo-org/matomo/issues/6784

Before working on this need to clarify with @mattab how to solve this problem. Related also https://github.com/matomo-org/matomo/issues/6784 which may need similar solution.