Plot Example

The Plot example shows how to display a graph from data using an opaque container.

It draws an sine graph using QPainter.drawPolyline() from a list of points. The list of points is continuously updated, as is the case for a example for a graph of an oscilloscope or medical patient monitor. In this case, it makes sense from a performance point of view to avoid the conversion of a Python list of data to a C++ list (QList<QPoint>) for each call to the plot function QPainter.drawPolyline(). This is where opaque containers come into play.

Instead of Python list of points, a QPointList is instantiated to store the data. QPointList is an opaque container wrapping a QList<QPoint>. It can be passed to QPainter.drawPolyline() instead of a Python list of points.

The type is declared in the entry for the QList container type in the type system file of the QtCore library:

<container-type name="QList" type="list"
                opaque-containers="int:QIntList;QPoint:QPointList;QPointF:QPointFList">
    ...
</container-type>

In the shift() member function, new data are appended to the list while old data moving out of the visible window are removed from the front of the list.

Download this example

plot.py

# Copyright (C) 2022 The Qt Company Ltd.
# SPDX-License-Identifier: LicenseRef-Qt-Commercial OR BSD-3-Clause

import math
import sys

from PySide6.QtWidgets import QWidget, QApplication
from PySide6.QtCore import QPoint, QRect, QTimer, Qt
from PySide6.QtGui import QPainter, QPointList


WIDTH = 680
HEIGHT = 480


class PlotWidget(QWidget):
    """Illustrates the use of opaque containers. QPointList
       wraps a C++ QList<QPoint> directly, removing the need to convert
       a Python list in each call to QPainter.drawPolyline()."""

    def __init__(self, parent=None):
        super().__init__(parent)
        self._timer = QTimer(self)
        self._timer.setInterval(20)
        self._timer.timeout.connect(self.shift)

        self._points = QPointList()
        self._points.reserve(WIDTH)
        self._x = 0
        self._delta_x = 0.05
        self._half_height = HEIGHT / 2
        self._factor = 0.8 * self._half_height

        for i in range(WIDTH):
            self._points.append(QPoint(i, self.next_point()))

        self.setFixedSize(WIDTH, HEIGHT)

        self._timer.start()

    def next_point(self):
        result = self._half_height - self._factor * math.sin(self._x)
        self._x += self._delta_x
        return result

    def shift(self):
        last_x = self._points[WIDTH - 1].x()
        self._points.pop_front()
        self._points.append(QPoint(last_x + 1, self.next_point()))
        self.update()

    def paintEvent(self, event):
        with QPainter(self) as painter:
            rect = QRect(QPoint(0, 0), self.size())
            painter.fillRect(rect, Qt.white)
            painter.translate(-self._points[0].x(), 0)
            painter.drawPolyline(self._points)


if __name__ == "__main__":

    app = QApplication(sys.argv)

    w = PlotWidget()
    w.show()
    sys.exit(app.exec())