Python/Matplotlib - Quickly Updating Text on Axes

Question

I have a matplotlib figure/canvas in a wxpython window. I want to update some information on the plot as the mouse moves around. I've connected to 'motion_notify_event' to get this information.

In the code below, a lot of random data is plotted and then the x,y location of the cursor is displayed in the statusbar of the window. This is very smooth and works well. However, I really want to display this information at the top of the plot. The behavior I want is shown if you uncomment the last two lines of cbUpdateCursor. However, when this is done, the response time to moving the cursor is terribly slow (because draw gets called and there is a lot of data, but draw must be called or the text doesn't get updated).

How can I speed this up so the cursor position can be displayed on the plot, but not slow it down so much? I think I might need to do something with bbox?

Code:

import wx
import numpy as np
import matplotlib
matplotlib.use('WXAgg')
from matplotlib.figure import Figure
from matplotlib.widgets import Cursor
from matplotlib.backends.backend_wxagg import \
   FigureCanvasWxAgg as FigCanvas, \
   NavigationToolbar2WxAgg as NavigationToolbar

class wxPlotting(wx.Frame):
   title = 'Test'
   def __init__(self):
      wx.Frame.__init__(self, None, -1, self.title)
      self.time = np.arange(10000)
      self.data = np.random.random(10000)
      self.sb = self.CreateStatusBar()
      self.create_main_panel()
      self.axes.plot(self.time, self.data)
      self.canvas.draw()

   def create_main_panel(self):
      self.panel = wx.Panel(self)
      self.fig = Figure((5.0, 4.0), dpi=100)
      self.canvas = FigCanvas(self.panel, -1, self.fig)
      self.axes = self.fig.add_subplot(111)
      self.text = self.axes.text(0., 1.005, '', transform = self.axes.transAxes)
      self.cursor = Cursor(self.axes, useblit=True, color='red')
      self.canvas.mpl_connect('motion_notify_event', self.cbUpdateCursor)
      self.vbox = wx.BoxSizer(wx.VERTICAL)
      self.vbox.Add(self.canvas, 1, wx.LEFT | wx.TOP | wx.GROW)
      self.panel.SetSizer(self.vbox)
      self.vbox.Fit(self)

   def cbUpdateCursor(self, event):
      if event.inaxes:
         text = 'x = %5.4f, y = %5.4f' % (event.xdata, event.ydata)
         self.sb.SetStatusText(text)
         #self.text.set_text(text)
         #self.canvas.draw()

if __name__ == '__main__':
   app = wx.PySimpleApp()
   app.frame = wxPlotting()
   app.frame.Show()
   app.MainLoop()

Basically I want something similar to the text that gets displayed using pyplot, i.e. the bottom right corner when the code below is run:

Code:

import matplotlib.pyplot as plt
plt.plot(range(10000), range(10000))
plt.show()

EDIT:

In my actual program, I want the static text to be within the matplotlib axes, not really above it. So I don't think I can just use a wxpython statictext to display it.

Answer 1

You could use blitting, similar to the animation examples here.

This make a very large performance difference in this case, as only a small portion of the window needs to be redrawn.

Unfortunately, I can't figure out how to get a gray background behind the text when it's redrawn, to match the default figure background behind it... The performance is excellent, though.

As a stand-alone example based on your code above:

import wx
import numpy as np
import matplotlib
matplotlib.use('WXAgg')
from matplotlib.figure import Figure
from matplotlib.widgets import Cursor
from matplotlib.backends.backend_wxagg import \
   FigureCanvasWxAgg as FigCanvas, \
   NavigationToolbar2WxAgg as NavigationToolbar

class wxPlotting(wx.Frame):
   title = 'Test'
   def __init__(self):
      wx.Frame.__init__(self, None, -1, self.title)
      self.time = np.arange(10000)
      self.data = np.random.random(10000)
      self.sb = self.CreateStatusBar()
      self.create_main_panel()
      self.axes.plot(self.time, self.data)
      self.background = self.canvas.copy_from_bbox(self.fig.bbox)
      self.canvas.draw()

   def create_main_panel(self):
      self.panel = wx.Panel(self)
      self.fig = Figure((5.0, 4.0), dpi=100)
      self.canvas = FigCanvas(self.panel, -1, self.fig)
      self.axes = self.fig.add_subplot(111)
      self.text = self.axes.text(0., 1.005, '', transform = self.axes.transAxes, animated=True)
      self.cursor = Cursor(self.axes, useblit=True, color='red')
      self.canvas.mpl_connect('motion_notify_event', self.cbUpdateCursor)
      self.vbox = wx.BoxSizer(wx.VERTICAL)
      self.vbox.Add(self.canvas, 1, wx.LEFT | wx.TOP | wx.GROW)
      self.panel.SetSizer(self.vbox)
      self.vbox.Fit(self)

   def cbUpdateCursor(self, event):
      if event.inaxes:
         text = 'x = %5.4f, y = %5.4f' % (event.xdata, event.ydata)
         self.sb.SetStatusText(text)

         self.canvas.restore_region(self.background)
         self.text.set_text(text)
         self.axes.draw_artist(self.text)
         self.canvas.blit(self.text.get_window_extent())

if __name__ == '__main__':
   app = wx.PySimpleApp()
   app.frame = wxPlotting()
   app.frame.Show()
   app.MainLoop()