Ytukyg

I want to show a given plot inside a PyQt application and show a line that moves along with the mouse cursor. However, the line shows up on the left of the plot and doesn't move at all when moving the cursor. I don't know why this is happening. Here's the code:

import numpy as np

from matplotlib.backends.qt_compat import QtWidgets

from matplotlib.backends.backend_qt5agg import FigureCanvas

from matplotlib.figure import Figure

import matplotlib.pyplot as plt



class SnaptoCursor(object):

    def __init__(self, ax, x):

        self.ax = ax

        self.ly = ax.axvline(color='k')

        self.x = x

        self.txt = ax.text(0.7, 0.9, '', transform=ax.transAxes)



    def mouse_move(self, event):

        if event.inaxes:

            indx = np.searchsorted(self.x, [event.xdata])[0]

            x = self.x[indx]

            self.ly.set_xdata(x)

            self.txt.set_text('x=%1.2f' % x)

            self.ax.figure.canvas.draw()

        else:

            pass



class App(QtWidgets.QMainWindow):         

    def __init__(self, parent=None):

        super(App, self).__init__(parent)



        self._main = QtWidgets.QWidget()

        self.setCentralWidget(self._main)

        self.figure = Figure(figsize=(10, 6.9))

        self.canvas = FigureCanvas(self.figure)

        self.canvas_ax = self.canvas.figure.subplots()

        x = np.arange(0,40)

        self.canvas_ax.plot(x, np.random.rand(40))



        # Layout

        layout = QtWidgets.QVBoxLayout(self._main)

        layout.addWidget(self.canvas)

        self.showMaximized()



        ##############################################

        ########## This doesnt seem to work ##########

        ##############################################

        cursor = SnaptoCursor(self.canvas_ax, x)

        plt.connect('motion_notify_event', cursor.mouse_move)

        ##############################################

        ##############################################

        ##############################################



if __name__ == '__main__':   

    app = QtWidgets.QApplication()   

    ex = App()

    ex.show()

    app.exec_()

The two lines in question should be

self.cursor = SnaptoCursor(self.canvas_ax, x)

self.cid = self.canvas.mpl_connect('motion_notify_event', self.cursor.mouse_move)

because else the cursor as well as the callback registration do not exist in memory at the point you would expect them to be used. And also you shouldn't be using pyplot/plt when your figure isn't even created with pyplot.