Add a switch to Qt Graphs version of DateTimeAxis to let people choose timezone

Back in Qt Charts, DateTimeAxis labels in local timezone. For example, a default DateTimeAxis has its origin at Jan. 1st, 1970, 09:00 if the system environment is in UTC+9. But in Qt Graphs, it is UTC 0 regardless, i.e. even though I am using it from UTC+9 the origin is still Jan. 1st, 1970, 00:00. It is said in documentation that "The DateTime is stored as UTC internally", yes. But it does not necessarily mean that DateTimeAxis has to use the raw UTC 0 timezone?

So adding a switch for people to choose either to use UTC 0 or local timezone can come in handy. Otherwise, people need to do calculation themselves.