Matplotlib cross hair cursor in PyQt5 - matplotlib

Question

I want to add a cross hair that snaps to data points and be updated on mouse move. I found this example that works well:
import numpy as np
import matplotlib.pyplot as plt
class SnappingCursor:
"""
A cross hair cursor that snaps to the data point of a line, which is
closest to the *x* position of the cursor.
For simplicity, this assumes that *x* values of the data are sorted.
"""
def __init__(self, ax, line):
self.ax = ax
self.horizontal_line = ax.axhline(color='k', lw=0.8, ls='--')
self.vertical_line = ax.axvline(color='k', lw=0.8, ls='--')
self.x, self.y = line.get_data()
self._last_index = None
# text location in axes coords
self.text = ax.text(0.72, 0.9, '', transform=ax.transAxes)
def set_cross_hair_visible(self, visible):
need_redraw = self.vertical_line.get_visible() != visible
self.vertical_line.set_visible(visible)
self.horizontal_line.set_visible(visible)
self.text.set_visible(visible)
return need_redraw
def on_mouse_move(self, event):
if not event.inaxes:
self._last_index = None
need_redraw = self.set_cross_hair_visible(False)
if need_redraw:
self.ax.figure.canvas.draw()
else:
self.set_cross_hair_visible(True)
x, y = event.xdata, event.ydata
index = min(np.searchsorted(self.y, y), len(self.y) - 1)
if index == self._last_index:
return # still on the same data point. Nothing to do.
self._last_index = index
x = self.x[index]
y = self.y[index]
# update the line positions
self.horizontal_line.set_ydata(y)
self.vertical_line.set_xdata(x)
self.text.set_text('x=%1.2f, y=%1.2f' % (x, y))
self.ax.figure.canvas.draw()
y = np.arange(0, 1, 0.01)
x = np.sin(2 * 2 * np.pi * y)
fig, ax = plt.subplots()
ax.set_title('Snapping cursor')
line, = ax.plot(x, y, 'o')
snap_cursor = SnappingCursor(ax, line)
fig.canvas.mpl_connect('motion_notify_event', snap_cursor.on_mouse_move)
plt.show()
But I get into trouble when I want to adapt the code with the PyQt5 and show the plot in a GUI. My code is:
from PyQt5.QtWidgets import QApplication, QMainWindow, QWidget, QVBoxLayout
import sys
from matplotlib.backends.backend_qt5agg import FigureCanvasQTAgg as FigureCanvas
from matplotlib.figure import Figure
import numpy as np
class SnappingCursor:
"""
A cross hair cursor that snaps to the data point of a line, which is
closest to the *x* position of the cursor.
For simplicity, this assumes that *x* values of the data are sorted.
"""
def __init__(self, ax, line):
self.ax = ax
self.horizontal_line = ax.axhline(color='k', lw=0.8, ls='--')
self.vertical_line = ax.axvline(color='k', lw=0.8, ls='--')
self.x, self.y = line.get_data()
self._last_index = None
# text location in axes coords
self.text = ax.text(0.72, 0.9, '', transform=ax.transAxes)
def set_cross_hair_visible(self, visible):
need_redraw = self.vertical_line.get_visible() != visible
self.vertical_line.set_visible(visible)
self.horizontal_line.set_visible(visible)
self.text.set_visible(visible)
return need_redraw
def on_mouse_move(self, event):
if not event.inaxes:
self._last_index = None
need_redraw = self.set_cross_hair_visible(False)
if need_redraw:
self.ax.figure.canvas.draw()
else:
self.set_cross_hair_visible(True)
x, y = event.xdata, event.ydata
index = min(np.searchsorted(self.y, y), len(self.y) - 1)
if index == self._last_index:
return # still on the same data point. Nothing to do.
self._last_index = index
x = self.x[index]
y = self.y[index]
# update the line positions
self.horizontal_line.set_ydata(y)
self.vertical_line.set_xdata(x)
self.text.set_text('x=%1.2f, y=%1.2f' % (x, y))
self.ax.figure.canvas.draw()
class Window(QMainWindow):
def __init__(self):
super().__init__()
widget=QWidget()
vbox=QVBoxLayout()
plot1 = FigureCanvas(Figure(tight_layout=True, linewidth=3))
ax = plot1.figure.subplots()
x = np.arange(0, 1, 0.01)
y = np.sin(2 * 2 * np.pi * x)
line, = ax.plot(x, y, 'o')
snap_cursor = SnappingCursor(ax, line)
plot1.mpl_connect('motion_notify_event', snap_cursor.on_mouse_move)
vbox.addWidget(plot1)
widget.setLayout(vbox)
self.setCentralWidget(widget)
self.setWindowTitle('Example')
self.show()
App = QApplication(sys.argv)
window = Window()
sys.exit(App.exec())
By running the above code, the data is plotted properly, but the cross hair is only shown in its initial position and does not move by mouse movement. Data values are also not displayed.
I have found a similar question here too, but the question is not answered clearly.

Answer 1

There are 2 problems:
snap_cursor is a local variable that will be removed when __init__ finishes executing. You must make him a member of the class.
The initial code of the tutorial is designed so that the point that information is displayed is the horizontal line that passes through the cursor and intersects the curve. In your initial code it differs from the example and also does not work for your new curve so I restored the logic of the tutorial.
import sys
from PyQt5.QtWidgets import QApplication, QMainWindow, QVBoxLayout, QWidget
import numpy as np
from matplotlib.backends.backend_qt5agg import FigureCanvasQTAgg as FigureCanvas
from matplotlib.figure import Figure
class SnappingCursor:
"""
A cross hair cursor that snaps to the data point of a line, which is
closest to the *x* position of the cursor.
For simplicity, this assumes that *x* values of the data are sorted.
"""
def __init__(self, ax, line):
self.ax = ax
self.horizontal_line = ax.axhline(color="k", lw=0.8, ls="--")
self.vertical_line = ax.axvline(color="k", lw=0.8, ls="--")
self.x, self.y = line.get_data()
self._last_index = None
# text location in axes coords
self.text = ax.text(0.72, 0.9, "", transform=ax.transAxes)
def set_cross_hair_visible(self, visible):
need_redraw = self.vertical_line.get_visible() != visible
self.vertical_line.set_visible(visible)
self.horizontal_line.set_visible(visible)
self.text.set_visible(visible)
return need_redraw
def on_mouse_move(self, event):
if not event.inaxes:
self._last_index = None
need_redraw = self.set_cross_hair_visible(False)
if need_redraw:
self.ax.figure.canvas.draw()
else:
self.set_cross_hair_visible(True)
x, y = event.xdata, event.ydata
index = min(np.searchsorted(self.x, x), len(self.x) - 1)
if index == self._last_index:
return # still on the same data point. Nothing to do.
self._last_index = index
x = self.x[index]
y = self.y[index]
# update the line positions
self.horizontal_line.set_ydata(y)
self.vertical_line.set_xdata(x)
self.text.set_text("x=%1.2f, y=%1.2f" % (x, y))
self.ax.figure.canvas.draw()
class Window(QMainWindow):
def __init__(self):
super().__init__()
widget = QWidget()
vbox = QVBoxLayout(widget)
x = np.arange(0, 1, 0.01)
y = np.sin(2 * 2 * np.pi * x)
canvas = FigureCanvas(Figure(tight_layout=True, linewidth=3))
ax = canvas.figure.subplots()
ax.set_title("Snapping cursor")
(line,) = ax.plot(x, y, "o")
self.snap_cursor = SnappingCursor(ax, line)
canvas.mpl_connect("motion_notify_event", self.snap_cursor.on_mouse_move)
vbox.addWidget(canvas)
self.setCentralWidget(widget)
self.setWindowTitle("Example")
app = QApplication(sys.argv)
w = Window()
w.show()
app.exec()