The personal website of Scott W Harden
⚠️ Warning: This article is obsolete.
Articles typically receive this designation when the technology they describe is no longer relevant, code provided is later deemed to be of poor quality, or the topics discussed are better presented in future articles. Articles like this are retained for the sake of preservation, but their content should be critically assessed.
Live Data in PyQt4 with PlotWidget
July 31st, 2016

Live Data in PyQt4 with PlotWidget

After spending a day comparing performance of MatplotlibWidget with PlotWidget, when it comes to speed PlotWidget wins by a mile! Glance over my last post where I describe how to set up the window with QT Designer and convert the .ui file to a .py file. With only a few changes to the code, I swapped the matplotlib _MatplotlibWidget _with the pyqtgraph PlotWidget. I easily got a 20x increase in speed (frame rate), and I'm likely to favor PyQtGraph over matpltolib for python applications involving realtime display of data. Just like the previous example using matplotlib, this one uses the system time to control the phase and color of a sine wave in real time. You can grab the full code from my github folder.

When designing the GUI with QT Designer, add a QGraphicsView widget then assign it to become a PyQtGraph object. To do this, follow the steps found on the pyqtgraph docs page:

  • In Designer, create a QGraphicsView widget
  • Right-click on the QGraphicsView and select “Promote To...”
  • Set “Promoted class name” to “PlotWidget”
  • Under “Header file”, enter “pyqtgraph”
  • Click “Add”, then click “Promote”
  • apparently this only needs to be done once per project

In addition to faster frame rate, the PyQtGraph method is easy to interact with. Clicking and dragging scrolls the data, and right-clicking and dragging zooms on each axis. Here's the program code:

from PyQt4 import QtGui,QtCore
import sys
import ui_main
import numpy as np
import pylab
import time
import pyqtgraph

class ExampleApp(QtGui.QMainWindow, ui_main.Ui_MainWindow):
    def __init__(self, parent=None):
        pyqtgraph.setConfigOption('background', 'w') #before loading widget
        super(ExampleApp, self).__init__(parent)
        self.setupUi(self)
        self.btnAdd.clicked.connect(self.update)
        self.grPlot.plotItem.showGrid(True, True, 0.7)

    def update(self):
        t1=time.clock()
        points=100 #number of data points
        X=np.arange(points)
        Y=np.sin(np.arange(points)/points*3*np.pi+time.time())
        C=pyqtgraph.hsvColor(time.time()/5%1,alpha=.5)
        pen=pyqtgraph.mkPen(color=C,width=10)
        self.grPlot.plot(X,Y,pen=pen,clear=True)
        print("update took %.02f ms"%((time.clock()-t1)*1000))
        if self.chkMore.isChecked():
            QtCore.QTimer.singleShot(1, self.update) # QUICKLY repeat

if __name__=="__main__":
    app = QtGui.QApplication(sys.argv)
    form = ExampleApp()
    form.show()
    form.update() #start with something
    app.exec_()
    print("DONE")

This project is available on GitHub: https://github.com/swharden/Python-GUI-examples/tree/master/2016-07-31_qt_PyQtGraph_sine_scroll

Newer: Python Real-time Audio Frequency Monitor
Older: Live Data in PyQt4 with MatplotlibWidget
All Blog Posts