Update and switch plots

Question

I am trying to update several plots using a matplotlibs embedding in a QT widget. Right now I can have one plot update in the window. But when I try to switch to the other plot by clicking a button the program freezes.

This is a test script I am using to understand how to use the programming tool to integrate into a larger program. I modified the code from this question: How to embed matplotlib in pyqt - for Dummies

I have been stuck on this problem for a while now. I know I am missing something very simple.

import random
import sys
from PyQt4 import QtGui, QtCore

from matplotlib.backends.backend_qt4agg import FigureCanvasQTAgg as FigureCanvas
from matplotlib.backends.backend_qt4agg import NavigationToolbar2QT as NavigationToolbar
from matplotlib.figure import Figure

class Window(QtGui.QDialog):
    def __init__(self, parent=None):
        super(Window, self).__init__(parent)

        # a figure instance to plot on
        self.figure1 = Figure()
        self.figure2 = Figure()
        self.current = "fig1"
        # this is the Canvas Widget that displays the `figure`
        # it takes the `figure` instance as a parameter to __init__
        self.canvas = FigureCanvas(self.figure1)
        self.ax1 = self.figure1.add_subplot(111)
        self.ax2 = self.figure2.add_subplot(111)
        self.line1, = self.ax1.plot([], [], 'r', lw=2)
        self.line2, = self.ax2.plot([], [], 'b', lw=2)
        # this is the Navigation widget
        # it takes the Canvas widget and a parent
        self.toolbar = NavigationToolbar(self.canvas, self)

        # Just some button connected to `plot` method
        self.button = QtGui.QPushButton('Plot')
        self.button.clicked.connect(self.plot)

        # set the layout
        layout = QtGui.QVBoxLayout()
        layout.addWidget(self.toolbar)
        layout.addWidget(self.canvas)
        layout.addWidget(self.button)
        self.setLayout(layout)
        self.update()


    def update(self):
        datax = [random.random() for i in range(10)]
        datay = [random.random() for i in range(10)]

        self.line1.set_xdata(datax)
        self.line1.set_ydata(datay)
        self.ax1.relim()
        self.ax1.autoscale_view()


        self.line2.set_xdata(datax)
        self.line2.set_ydata(datay)
        self.ax2.relim()
        self.ax2.autoscale_view()
        self.canvas.draw()
        QtCore.QTimer.singleShot(1, self.update)




    def plot(self):

        if self.current == "fig1":
            self.canvas = FigureCanvas(self.figure2)
            self.current = "fig2"
        elif self.current == "fig2":
            self.canvas = FigureCanvas(self.figure1)
            self.current = "fig1"


if __name__ == '__main__':
    app = QtGui.QApplication(sys.argv)

    main = Window()
    main.show()

    sys.exit(app.exec_())

When i click this button it should start plotting the other plot. I get no error messages.

Answer 1

Your logic of making the canvas change:

def plot(self):
    if self.current == "fig1":
        self.canvas = FigureCanvas(self.figure2)
        self.current = "fig2"
    elif self.current == "fig2":
        self.canvas = FigureCanvas(self.figure1)
        self.current = "fig1"

It is similar to the following to the following logic:

class Foo:
    def __init__(self):
        self.m_a = 0

    def setA(self, a):
        self.m_a = a

    def printA(self):
        print(self.m_a)


class Bar:
    def __init__(self):
        self.a = 5
        self.foo = Foo()
        self.foo.setA(self.a)

    def change(self):
        self.a = 6


if __name__ == "__main__":
    bar = Bar()
    bar.foo.printA()
    bar.change()
    bar.foo.printA()

And if the code is executed you get the following:

5
5

As you see a change of a variable does not imply that the other classes are notified because the name of a variable is a nickname, the important thing is the memory, in your case the displayed canvas occupies a memory place different from the canvas that you create when you press the button, the same happens with line1, line2, ax1 and ax2 that are associated with the initial canvas.

---

On the other hand in a GUI is not recommended to delete and create widgets, instead it is better to hide a widget and show the other that occupies the same space and for this we can use a QStackedWidget or QStackedLayout.

import sys
import random
from PyQt4 import QtGui, QtCore

from matplotlib.backends.backend_qt4agg import FigureCanvasQTAgg as FigureCanvas
from matplotlib.backends.backend_qt4agg import (
    NavigationToolbar2QT as NavigationToolbar,
)
from matplotlib.figure import Figure


class Window(QtGui.QDialog):
    def __init__(self, parent=None):
        super(Window, self).__init__(parent)

        self.figure1 = Figure()
        self.canvas1 = FigureCanvas(self.figure1)
        self.ax1 = self.figure1.add_subplot(111)
        self.line1, = self.ax1.plot([], [], "r", lw=2)
        self.toolbar1 = NavigationToolbar(self.canvas1, self)

        self.figure2 = Figure()
        self.canvas2 = FigureCanvas(self.figure2)
        self.ax2 = self.figure2.add_subplot(111)
        self.line2, = self.ax2.plot([], [], "b", lw=2)
        self.toolbar2 = NavigationToolbar(self.canvas2, self)

        self.m_stacked_layout = QtGui.QStackedLayout()

        for canvas, toolbar in (
            (self.canvas1, self.toolbar1),
            (self.canvas2, self.toolbar2),
        ):
            widget = QtGui.QWidget()
            lay = QtGui.QVBoxLayout(widget)
            lay.addWidget(toolbar)
            lay.addWidget(canvas)
            self.m_stacked_layout.addWidget(widget)

        self.button = QtGui.QPushButton("Plot", clicked=self.onClicled)

        layout = QtGui.QVBoxLayout(self)
        layout.addLayout(self.m_stacked_layout)
        layout.addWidget(self.button)

        timer = QtCore.QTimer(self, timeout=self.update_plot, interval=1)
        timer.start()
        self.update_plot()

    @QtCore.pyqtSlot()
    def update_plot(self):
        datax = [random.random() for i in range(10)]
        datay = [random.random() for i in range(10)]

        self.line1.set_xdata(datax)
        self.line1.set_ydata(datay)
        self.ax1.relim()
        self.ax1.autoscale_view()

        self.line2.set_xdata(datax)
        self.line2.set_ydata(datay)
        self.ax2.relim()
        self.ax2.autoscale_view()
        self.canvas1.draw()
        self.canvas2.draw()

    @QtCore.pyqtSlot()
    def onClicled(self):
        ix = self.m_stacked_layout.currentIndex()
        self.m_stacked_layout.setCurrentIndex(0 if ix == 1 else 1)


if __name__ == "__main__":
    app = QtGui.QApplication(sys.argv)

    main = Window()
    main.show()

    sys.exit(app.exec_())