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matplotlib crosshair cursor in PyQt dialog does not show up

I want to have a crosshair at the cursor in my matplotlib window. This works out in the example, given in the matplotlib gallery. But unfortunately, it does not work if I have the matplotlib widget in a Qt dialog window (QDialog).

matplotlib

QDialog

This is my sample of code, where I want to instantiate the matplotlib.widgets.Cursor object, but nothing shows up.

matplotlib.widgets.Cursor

import sys
from PyQt4 import QtGui
from matplotlib.backends.backend_qt4agg import FigureCanvasQTAgg as FigureCanvas
import matplotlib.pyplot as plt
from matplotlib.widgets import Cursor

class Window(QtGui.QDialog):
def __init__(self, parent=None):
super(Window, self).__init__(parent)
self.figure = plt.figure(facecolor='white')
self.canvas = FigureCanvas(self.figure)
layout = QtGui.QVBoxLayout()
layout.addWidget(self.canvas)
self.setLayout(layout)

''' plot some random stuff '''
ax = self.figure.add_subplot(111)
self.ax = ax
ax.plot([1,2])

# Set cursor
Cursor(self.ax, useblit=False, color='red', linewidth=1)
self.canvas.draw()

if __name__ == '__main__':
app = QtGui.QApplication(sys.argv)
main = Window()
main.show()
sys.exit(app.exec_())


Can someone help?

2 Answers
2

I have modified your code as follows and it works on my computer. Hope it helps.

import sys
from PyQt4 import QtGui
from matplotlib.backends.backend_qt4agg import FigureCanvasQTAgg as FigureCanvas
import matplotlib.pyplot as plt
from matplotlib.widgets import Cursor

class Window(QtGui.QDialog):
def __init__(self, parent=None):
super(Window, self).__init__(parent)
self.figure = plt.figure(facecolor='white')
self.canvas = FigureCanvas(self.figure)
layout = QtGui.QVBoxLayout()
layout.addWidget(self.canvas)
self.setLayout(layout)

''' plot some random stuff '''
ax = self.figure.add_subplot(111)
self.ax = ax
ax.plot([1,2])
# Set cursor
cursor = Cursor(self.ax, useblit=False, color='red', linewidth=1)

############## The added part: #############
def onclick(event):
cursor.onmove(event)
self.canvas.mpl_connect('button_press_event', onclick)
############################################
self.canvas.draw()

if __name__ == '__main__':
app = QtGui.QApplication(sys.argv)
main = Window()
main.show()
sys.exit(app.exec_())


The original code in (1):
Cursor(self.ax, useblit=False, color='red', linewidth=1)
don't leave a reference to the Cursor as indicated in the documentation.

The code in (2) has the assignment but not to a class reference but to a local variable:
cursor = Cursor(self.ax, useblit=False, color='red', linewidth=1)

The code in (2) works with the indicated additions but the cursor move slowly.

The solution is the code as in (1) but modified as follows:
self.cursor = Cursor(self.ax, useblit=False, color='red', linewidth=1)

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