Add Boxplot feature

[noonchen]

Add BoxplotItem and example code.

Closes #2542 .

[pijyoi]

I think you left out pyqtgraph/__init__.py
Also please add your example to pyqtgraph/examples/utils.py so that it gets added to ExampleApp and also gets executed by the CI.

From #2561, I found that getting the bounding rectangle right is not so straightforward. Below is a script that demonstrates two issues.

1. the size of the outlier symbol is not accounted for
2. when a thick pen is used, QPicture.boundingRect() computes the bounding rectangle wrongly

import numpy as np
import pyqtgraph as pg
from pyqtgraph.Qt import QtCore, QtGui, QtWidgets

class ObjectBounds(QtWidgets.QGraphicsItem):
    def paint(self, painter, *args):
        pen = QtGui.QPen(QtCore.Qt.GlobalColor.red, 0, QtCore.Qt.PenStyle.DashLine)
        rect = self.boundingRect()
        painter.setPen(pen)
        painter.drawRect(rect)
        print(rect)

    def boundingRect(self):
        return self.parentItem().boundingRect()

pg.mkQApp()
pw = pg.PlotWidget()
pw.show()

np.random.seed(8)
n = 5
data = [np.random.normal(500, 30, 1000) for _ in range(n)]

bpi = pg.BoxplotItem()
pen = pg.mkPen('y', width=6)
bpi.setData(data=data, pen=pen, symbol='star', symbolBrush='g')
pw.addItem(bpi)
rect = ObjectBounds(bpi)

pg.exec()

[noonchen]

@pijyoi Thanks for pointing out these issues, let me try to fix them.

[noonchen]

@pijyoi bounding rect should be correct now.

[pijyoi]

I am getting the plot below on Windows/PyQt6 and also on WSL2/PySide6. Note that there is no big empty space.
Did you paste an older screenshot?

[noonchen]

No, it's the screenshot from latest changes. I can't explain why there are still blank spaces surround bounding rect.

I'm running on macos + pyqt5

[pijyoi]

Ah, I see that the bounding rectangle is tight but the viewbox is not in your screenshot.
Perhaps this is platform specific.

Could you try out the example in #2561 on both #2561 and #2565 on your macOS system and see if there are any differences?
On {Windows,Linux} x {Qt5,Qt6} they look the same to me, i.e. no empty space.

Or even better, try out the script in #2565 (comment)

[pijyoi]

I think I made a mistake, deviceTransform should be used instead of mapToScene.
But if we follow ScatterPlotItem's code, we can just use QPainter.transform instead

        tr = p.transform()
        for pos, outliers in self.outlierData.items():
            for o in outliers:
                x, y = (pos, o) if self.opts["locAsX"] else (o, pos)
                p.resetTransform()
                p.translate(*tr.map(x, y))
                p.scale(symbolSize, symbolSize)
                p.drawPath(symbol)