How do I make matplotlib open a box for user comments?

Matplotlib will soon has now introduced a TextBox Widget. See it’s usage in this example.

Alternatively, you can use Tkinter’s tkSimpleDialog to ask the user for a comment.

w = tkSimpleDialog.askstring("Title", "Please type comment")

You may then annotate the last picked point with the comment.

Complete example (which runs in python 2.7):

import numpy as np
import matplotlib.pyplot as plt
import Tkinter, tkSimpleDialog

xy = [(0,0)]

def onpick(event):
    ''' '''
    if event.mouseevent.button == 1: #only want lmb clicks
        selection = event.artist
        xdata = selection.get_xdata()
        ydata = selection.get_ydata()
        ind = event.ind
        point = tuple(zip(xdata[ind], ydata[ind]))
        xclick,yclick = point[0] 
        xy[0] = (xclick,yclick)
        print('[x,y]=',xclick,yclick)   

def on_key(event):
    print('Key press:\'%s\'' %(event.key))
    if event.key == 'c':
        root = Tkinter.Tk()
        root.withdraw()
        w = tkSimpleDialog.askstring("Title", "Please type comment")
        if w != None:
            ax.annotate(w, xy=xy[0], xytext=(20,-20), 
                    arrowprops=dict(facecolor="black", width=2, headwidth=6),
                    textcoords="offset points")
            ax.figure.canvas.draw_idle()

fig = plt.figure(figsize=(8, 6))
ax = fig.add_subplot(111, facecolor="#FFFFCC")

x, y = 4*(np.random.rand(2, 100) - .5)
ax.plot(x, y, 'o', picker = 6)
ax.set_xlim(-2, 2)
ax.set_ylim(-2, 2)

keyID = fig.canvas.mpl_connect('key_press_event', on_key)
clickID = fig.canvas.mpl_connect('pick_event', onpick)

plt.show()