How to stop Matplotlib's navigation toolbar zoom from reseting on plot update?

Question

I have a hopefully relatively simple situation using Matplotlib's Navigation Toolbar. I want to be able to keep the previous zoom value, camera panning, etc. in between figure updates. I've kept PyQt5 embedding here (which I'm using in my project) in case there's some additional interlink needed between the two. Thank you so much for taking a look!

import sys
import os
import random
import matplotlib
matplotlib.use('Qt5Agg')
from PyQt5 import QtCore, QtWidgets
from PyQt5.QtWidgets import QGridLayout, QFileDialog, QPushButton

from numpy import arange
from matplotlib.backends.backend_qt5agg import FigureCanvasQTAgg as FigureCanvas
from matplotlib.backends.backend_qt5agg import NavigationToolbar2QT as NavigationToolbar
from matplotlib.figure import Figure


class MyMplCanvas(FigureCanvas):

    def __init__(self, parent=None, width=5, height=4, dpi=100):
        fig = Figure(figsize=(width, height), dpi=dpi)
        self.fig = fig ###
        self.axes = fig.add_subplot(111)

        self.axes.hold(False)

        self.compute_initial_figure()

        FigureCanvas.__init__(self, fig)
        self.setParent(parent)

        FigureCanvas.setSizePolicy(self,
                                   QtWidgets.QSizePolicy.Expanding,
                                   QtWidgets.QSizePolicy.Expanding)
        FigureCanvas.updateGeometry(self)

    def compute_initial_figure(self):
        pass


class MyDynamicMplCanvas(MyMplCanvas):
    def __init__(self, *args, **kwargs):
        MyMplCanvas.__init__(self, *args, **kwargs)
        timer = QtCore.QTimer(self)
        timer.timeout.connect(self.update_figure)
        timer.start(1000)

    def compute_initial_figure(self):
        self.axes.plot([0, 1, 2, 3], [1, 2, 0, 4], 'b')

    def update_figure(self):
        l = [random.randint(0, 10) for i in range(4)]
        self.axes.cla()
        self.axes.plot([0, 1, 2, 3], l, 'b')
        self.draw()


class P1(QtWidgets.QWidget):

    def __init__(self, parent=None):
        super(P1, self).__init__(parent)
        layout = QGridLayout(self)

        self.plot_canvas = MyDynamicMplCanvas(self, width=5, height=4, dpi=100)
        self.navi_toolbar = NavigationToolbar(self.plot_canvas, self)


        layout.addWidget(self.plot_canvas, 1, 1, 1, 1)
        layout.addWidget(self.navi_toolbar, 2, 1, 1, 1)



class MainWindow(QtWidgets.QMainWindow):    
    def __init__(self, parent=None):
        super(MainWindow, self).__init__(parent)
        self.setAttribute(QtCore.Qt.WA_DeleteOnClose)

        self.stack = QtWidgets.QStackedWidget(self)
        P1f = P1(self)
        self.stack.addWidget(P1f)
        self.setCentralWidget(self.stack)


if __name__ == '__main__':
    qApp = QtWidgets.QApplication(sys.argv)
    aw = MainWindow()
    aw.show()
    sys.exit(qApp.exec_())

Gabriel Jablonski · Accepted Answer · 2018-06-15 23:07:26Z

2

The big villains here are axes.hold(False) and axes.cla(). They are responsible for clearing the axes and figure, which (generally) will reset the plot view.

With those out of the way, you can properly use self.axes.autoscale(enable=False), which I recommend you put after your first plot in compute_initial_figure(), so the plot is at least somewhat scaled at the start.

Then, to be able to clear your previous plots, you can create another attribute for the MyMplCanvas class, maybe self.plotted_line or something like that, initialized with None. Everytime you call self.axes.plot(...), assign the return value to self.plotted_line, like so: self.plotted_line, = self.axes.plot(...). Notice the comma after self.plotted_line, which is one of the ways to assign only the first return value, which is the one you're interested in.

Lastly, right before each new plot, check and call

    if self.plotted_line is not None:
        self.plot.remove()

which will effectively remove the previous plot.

The canvas classes will look something like this (changes were minor).

class MyMplCanvas(FigureCanvas):
    def __init__(self, parent=None, width=5, height=4, dpi=100):
        fig = Figure(figsize=(width, height), dpi=dpi)
        self.fig = fig ###
        self.axes = fig.add_subplot(111)

        self.plotted_line = None

        self.compute_initial_figure()

        FigureCanvas.__init__(self, fig)
        self.setParent(parent)

        FigureCanvas.setSizePolicy(self,
                                   QtWidgets.QSizePolicy.Expanding,
                                   QtWidgets.QSizePolicy.Expanding)
        FigureCanvas.updateGeometry(self)

    def compute_initial_figure(self):
        pass


class MyDynamicMplCanvas(MyMplCanvas):
    def __init__(self, *args, **kwargs):
        MyMplCanvas.__init__(self, *args, **kwargs)
        timer = QtCore.QTimer(self)
        timer.timeout.connect(self.update_figure)
        timer.start(1000)

    def compute_initial_figure(self):
        self.plotted_line, = self.axes.plot([0, 1, 2, 3], [1, 2, 0, 4], 'b')
        self.axes.autoscale(enable=False)

    def update_figure(self):
        l = [random.randint(0, 10) for i in range(4)]

        if self.plotted_line is not None:
            self.plotted_line.remove()
        self.plotted_line, = self.axes.plot([0, 1, 2, 3], l, 'b')
        self.draw()

edited Jun 15, 2018 at 23:07

answered Jun 15, 2018 at 20:48

Gabriel Jablonski

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2 Comments

ees Over a year ago

Thank you so much @Gabriel! I'm sorry for the delay in response... I feel really bad, but although this fixes my example, I can't actually get this to work with my real program (which uses imshow with extra scatter points plotted on top). I may create a new topic just for a more detailed approach, but thank you so much for your time and help; your technique was very illuminating on the grander scale!

Gabriel Jablonski Over a year ago

You may indeed be able to use this, maybe by labeling related plots accordingly, so don't lose hope on this method just yet. Consider creating another post for sure.

bzu · Accepted Answer · 2019-04-14 14:23:18Z

The zoom values can be directly stored by connecting mpl 'draw_event' with:

def __init__(self, parent=None, width=5, height=4, dpi=100):
    ...
    self.mpl_connect('draw_event', self.on_draw)

def on_draw(self, event):
    self.xlim = self.axes.get_xlim()
    self.ylim = self.axes.get_ylim()

and then restored after plot() call:

def update_figure(self):
    ...
    self.axes.plot([0, 1, 2, 3], l, 'b')
    self.axes.set_xlim(self.xlim)
    self.axes.set_ylim(self.ylim)
    ...

https://matplotlib.org/users/event_handling.html

Full code:

import random
import sys

import matplotlib

matplotlib.use('Qt5Agg')
from PyQt5 import QtCore, QtWidgets
from PyQt5.QtWidgets import QGridLayout

from matplotlib.backends.backend_qt5agg import FigureCanvasQTAgg as FigureCanvas
from matplotlib.backends.backend_qt5agg import NavigationToolbar2QT as NavigationToolbar
from matplotlib.figure import Figure


class MyMplCanvas(FigureCanvas):

    def __init__(self, parent=None, width=5, height=4, dpi=100):
        fig = Figure(figsize=(width, height), dpi=dpi)
        self.fig = fig  ###
        self.axes = fig.add_subplot(111)

        self.axes.hold(False)

        self.compute_initial_figure()

        FigureCanvas.__init__(self, fig)
        self.setParent(parent)

        FigureCanvas.setSizePolicy(self,
                                   QtWidgets.QSizePolicy.Expanding,
                                   QtWidgets.QSizePolicy.Expanding)
        FigureCanvas.updateGeometry(self)
        self.xlim = self.axes.get_xlim()
        self.ylim = self.axes.get_ylim()
        self.mpl_connect('draw_event', self.on_draw)

    def on_draw(self, event):
        self.xlim = self.axes.get_xlim()
        self.ylim = self.axes.get_ylim()

    def compute_initial_figure(self):
        pass


class MyDynamicMplCanvas(MyMplCanvas):
    def __init__(self, *args, **kwargs):
        MyMplCanvas.__init__(self, *args, **kwargs)
        timer = QtCore.QTimer(self)
        timer.timeout.connect(self.update_figure)
        timer.start(1000)

    def compute_initial_figure(self):
        self.axes.plot([0, 1, 2, 3], [1, 2, 0, 4], 'b')

    def update_figure(self):
        l = [random.randint(0, 10) for i in range(4)]
        self.axes.cla()
        self.axes.plot([0, 1, 2, 3], l, 'b')
        self.axes.set_xlim(self.xlim)
        self.axes.set_ylim(self.ylim)
        self.draw()


class P1(QtWidgets.QWidget):

    def __init__(self, parent=None):
        super(P1, self).__init__(parent)
        layout = QGridLayout(self)

        self.plot_canvas = MyDynamicMplCanvas(self, width=5, height=4, dpi=100)
        self.navi_toolbar = NavigationToolbar(self.plot_canvas, self)

        layout.addWidget(self.plot_canvas, 1, 1, 1, 1)
        layout.addWidget(self.navi_toolbar, 2, 1, 1, 1)


class MainWindow(QtWidgets.QMainWindow):
    def __init__(self, parent=None):
        super(MainWindow, self).__init__(parent)
        self.setAttribute(QtCore.Qt.WA_DeleteOnClose)

        self.stack = QtWidgets.QStackedWidget(self)
        P1f = P1(self)
        self.stack.addWidget(P1f)
        self.setCentralWidget(self.stack)


if __name__ == '__main__':
    qApp = QtWidgets.QApplication(sys.argv)
    aw = MainWindow()
    aw.show()
    sys.exit(qApp.exec_())

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