如何以防止xdata变为None的方式更新matplotlib图形？

Question

I'm writing an application that periodically (approximately once per second) collects some data from a hardware device and displays it in a graph using matplotlib. The user will periodically click on the graph, and when that happens I need to know the x and y coordinates of the click, as measured in the graph's coordinate system.

This is easy enough to do: if canvas is a FigureCanvas instance, you can say

canvas.mpl_connect('button_release_event', mouse_up_handler)

and define

def mouse_up_handler(event):
    print('The user clicked at ({}, {})'.format(event.xdata, event.ydata))

to show the click coordinates in the graph's coordinate system. Occasionally, though, the user will click right as the graph is updating, and then event.xdata and .ydata are None . Right now I'm just discarding those clicks, but obviously it's pretty frustrating for the user when the application just ignores their clicks.

I've attached a full sample program so that you can see how I'm replacing the current graph with the new one. (If you run it and click on the graph repeatedly you'll see that the reported x coordinate is sometimes None .) Can I replace the figure's data in a way that never gives None for the coordinates? I only want one set of data to be shown at a time.

Example program

from __future__ import print_function
from math import exp
from matplotlib.backends.backend_wxagg import FigureCanvasWxAgg \
    as FigureCanvas
from matplotlib.figure import Figure
from random import normalvariate
from threading import Timer
from time import sleep
import wx

class GraphView(wx.Panel):
    def __init__(self, parent):
        wx.Panel.__init__(self, parent)

        self.figure = Figure()
        canvas = FigureCanvas(self, -1, self.figure)
        canvas.mpl_connect('button_release_event', self.mouse_up_handler)

        self.axes = None

        sizer = wx.BoxSizer(wx.VERTICAL)
        sizer.Add(canvas, 1, wx.GROW)
        self.SetSizer(sizer)

        self.generate_new_data()

    def generate_new_data(self):
        x_values = range(16384)
        y_values = [exp(normalvariate(0, 10)) for _ in range(16384)]

        if self.axes is not None:
            self.figure.delaxes(self.axes)

        sleep(0.4)  # simulate some additional work being done with the data

        self.axes = self.figure.add_subplot(1, 1, 1)
        self.axes.set_yscale('log')
        self.axes.autoscale(enable=True)
        self.axes.set_xlim(x_values[0], x_values[-1])
        self.axes.plot(x_values, y_values, antialiased=False)

        self.figure.canvas.draw()

        Timer(1.0, self.generate_new_data).start()

    def mouse_up_handler(self, event):
        print('Click at x = {}'.format(event.xdata))


class GraphWindow(wx.Frame):
    def __init__(self, parent):
        wx.Frame.__init__(self, parent)
        graph_view = GraphView(self)


if __name__ == '__main__':
    app = wx.App(False)
    GraphWindow(None).Show()
    app.MainLoop()

Answer 1

Something like:

def __init__(self, parent):
    wx.Panel.__init__(self, parent)

    self.figure = Figure()
    canvas = FigureCanvas(self, -1, self.figure)
    canvas.mpl_connect('button_release_event', self.mouse_up_handler)

    # make axes
    self.ax = self.figure.add_subplot(1, 1, 1)
    # set up details
    self.ax.set_yscale('log')
    self.ax.autoscale(enable=True)
    self.ax.set_xlim(x_values[0], x_values[-1]) 
    # make the line
    self.ln, = self.ax.plot([], []) # add what ever parameters you want

    sizer = wx.BoxSizer(wx.VERTICAL)
    sizer.Add(canvas, 1, wx.GROW)
    self.SetSizer(sizer)

    self.generate_new_data()



def generate_new_data(self):
    x_values = range(16384)
    y_values = [exp(normalvariate(0, 10)) for _ in range(16384)]

    sleep(0.4)  # simulate some additional work being done with the data

    # update your existing line
    self.ln.set_data(x_values, y_values, antialiased=False)

    self.figure.canvas.draw()

    Timer(1.0, self.generate_new_data).start()

to re-use the axes and Line2D objects.