Short description

Since #2011, thick pen plotting has become more usable. Plot Options -> Average uses a pen that is not supported by #2011, resulting in a very slow plot.

Code to reproduce

Run examples/crosshair.py.
On the upper plot, right-click -> context menu -> Plot Options -> Average -> check average

Expected behavior

No slow-down

Real behavior

Panning (amongst other operations) will become very slow, to the extent of being unusable.

Tested environment(s)

• PyQtGraph version: git ef458d3
• Qt Python binding: PyQt6 6.2.0
• Python version: 3.10.0
• NumPy version: 1.21.3
• Operating system: Windows 10
• Installation method: pip install -e .

Additional context

In pyqtgraph/graphicsItems/PlotItem/PlotItem.py, a shadow pen of color (0, 0, 0, 100), width=3 is created. Non-opaque pens are not supported by #2011, hence the slow-down.

Not sure what visual effect a non-opaque black pen is supposed to achieve.