Auto skipping allows a zoom level to skip ticks automatically if the
maximum number of ticks/pt is exceeded

Use new DateAxisItem in Plot Customization example

…ded the first one into utils.py

Replaces "DateAxisItem.attachToPlotItem"

Refactoring and slightly more documentation

This reverts commit 876b5a7.

Revisit last bug fix

Very large years (|y|>1e13) cause infinite loop, and since nobody
needs time 100 times larger than the age of the universe anyways,
this constrains it to 1e12.

Following suggestion by @axil:
pyqtgraph#1154 (comment)

While zooming out, before hitting OverflowErrors, utctimestamp
produces ValueErrors (at least on my Linux machine), so they
are also catched.

For large years, x axis labels jump by 1970 years if it is not
accounted for timestamp 0 to be equal to year 1970.

This commit catches the OSError like the other observed errors

2 reasons: First: At least on my Linux machine, zooming into
those dates creates infinite loops. Second: Nobody needs
sub-year-precision for those extreme years anyways.

DateTimeAxis: Hardcoded limit, catch ValueErrors under Linux, timestamp offset fix

This is somewhat experimental. With this commit, no longer 60 px are
assumed as width for all zoom levels, but the current font and
display resolution are considered to calculate the width of ticks in
each zoom level. See the new function `updateZoomLevels` for
details.
Before calling this function, overridden functions `paint` and
`generateDrawSpecs` provide information over the current display
and font via `self.fontScaleFactor` and `self.fontMetrics`.

As @axil noted in the DateAxisItem PR, currently users get a
segmentation fault when one tries to add an axis to multiple
PlotItems. This commit adds a meaningful RuntimeError message
for that case.

Meaningful error meassage when adding axis to multiple PlotItems

…n the fly

Adapt zoom level sizes based on current font size and screen resolution

This is both a simplification in code and an improvement in backwards compatibility with Qt 4.

…ility

Fix: Get screen resolution in a way that also works for Qt 4

DateAxisItem Fix: Also resolve time below 0.5 seconds

Stepping was off by 1970 years for years < 1 and > 9999,
resulting in a gap in ticks visible when zooming out. Fixed by
subtracting the usual 1970 years.

Fixed by setting default limits to +/- 1e10 years. Should still
be enough.

DateTimeAxis Fix: For years slightly > 9999, stepping broke

OrderedDict, correct y axis spacing