Dynamically update multiple axis in matplotlib

Question

I want to display sensor data on a PyQT GUI with a matplotlib animation. I already have a working Plot which gets updates every time I receive new sensor value from an external source with this code:

    def __init__(self):
            self.fig = Figure(figsize=(width, height), dpi=dpi)
            self.axes = self.fig.add_subplot(111)
            self.axes.grid()
            self.xdata = []
            self.ydata = []
            self.entry_limit = 50
            self.line, = self.axes.plot([0], [0], 'r')

    def update_figure_with_new_value(self, xval: float, yval: float):
            self.xdata.append(xval)
            self.ydata.append(yval)

            if len(self.xdata) > self.entry_limit:
                self.xdata.pop(0)
                self.ydata.pop(0)

            self.line.set_data(self.xdata, self.ydata)
            self.axes.relim()
            self.axes.autoscale_view()
            self.fig.canvas.draw()
            self.fig.canvas.flush_events()

I want now to extend the plot to show another data series with the same x-axis. I tried to achieve this with the following additions to the init-code above:

            self.axes2 = self.axes.twinx()
            self.y2data = []
            self.line2, = self.axes2.plot([0], [0], 'b')

and in the update_figure_with_new_value() function (for test purpose I just tried to add 1 to yval, I will extend the params of the function later):

            self.y2data.append(yval+1)
            if len(self.y2data) > self.entry_limit:
                self.y2data.pop(0)
            self.line2.set_data(self.xdata, self.ydata)
            self.axes2.relim()
            self.axes2.autoscale_view()

But instead of getting two lines in the plot which should have the exact same movement but just shifted by one I get vertical lines for the second plot axis (blue). The first axis (red) remains unchanged and is ok.

How can I use matplotlib to update multiple axis so that they display the right values?

I'm using python 3.4.0 with matplotlib 2.0.0.

Answer 1

Since there is no minimal example available, it's hard to tell the reason for this undesired behaviour. In principle ax.relim() and ax.autoscale_view() should do what you need.

So here is a complete example which works fine and updates both scales when being run with python 2.7, matplotlib 2.0 and PyQt4:

import numpy as np
import matplotlib.pyplot as plt
from PyQt4 import QtGui, QtCore
from matplotlib.figure import Figure
from matplotlib.backends.backend_qt4agg import FigureCanvasQTAgg as FigureCanvas
from matplotlib.backends.backend_qt4agg import NavigationToolbar2QT as NavigationToolbar

class Window(QtGui.QMainWindow):
    def __init__(self):
        QtGui.QMainWindow.__init__(self)
        self.widget = QtGui.QWidget()
        self.setCentralWidget(self.widget)
        self.widget.setLayout(QtGui.QVBoxLayout())
        self.widget.layout().setContentsMargins(0,0,0,0)
        self.widget.layout().setSpacing(0)

        self.fig = Figure(figsize=(5,4), dpi=100)
        self.axes = self.fig.add_subplot(111)
        self.axes.grid()
        self.xdata = [0]
        self.ydata = [0]
        self.entry_limit = 50
        self.line, = self.axes.plot([], [], 'r', lw=3)

        self.axes2 = self.axes.twinx()
        self.y2data = [0]
        self.line2, = self.axes2.plot([], [], 'b')

        self.canvas = FigureCanvas(self.fig)
        self.canvas.draw()

        self.nav = NavigationToolbar(self.canvas, self.widget)
        self.widget.layout().addWidget(self.nav)
        self.widget.layout().addWidget(self.canvas)

        self.show()

        self.ctimer = QtCore.QTimer()
        self.ctimer.timeout.connect(self.update)
        self.ctimer.start(150)


    def update(self):
        y = np.random.rand(1)
        self.update_figure_with_new_value(self.xdata[-1]+1,y)

    def update_figure_with_new_value(self, xval,yval):
        self.xdata.append(xval)
        self.ydata.append(yval)

        if len(self.xdata) > self.entry_limit:
            self.xdata.pop(0)
            self.ydata.pop(0)
            self.y2data.pop(0)

        self.line.set_data(self.xdata, self.ydata)
        self.axes.relim()
        self.axes.autoscale_view()

        self.y2data.append(yval+np.random.rand(1)*0.17)

        self.line2.set_data(self.xdata, self.y2data)
        self.axes2.relim()
        self.axes2.autoscale_view()

        self.fig.canvas.draw()
        self.fig.canvas.flush_events()


if __name__ == "__main__":
    qapp = QtGui.QApplication([])
    a = Window()
    exit(qapp.exec_())

You may want to test this and report back if it is working or not.