How to refresh text in Matplotlib?

Question

I wrote this code to read data from an excel file and to plot them. For a certain x-value, I desire to know the y-values of all the lines so I create a slider to change this x-v

Answer 1

With matplotlib widgets, the update method works best if you create an artist object and adjust its value (like you do with line in your code). For a text object, you can use the set_text and set_position method to change what it displays and where. As an example,

import numpy as np
import pylab as plt
from matplotlib.widgets import Slider

fig, ax = plt.subplots()
plt.subplots_adjust(bottom=0.25)
sax = plt.axes([0.25, 0.1, 0.65, 0.03])

x = np.linspace(0,2.*np.pi,100)
f = np.sin(x)
l, = ax.plot(x, f)

slider1 = Slider(sax, 'amplitude', -0.8, 0.8, valinit=0.8)

tpos = int(0.25*x.shape[0])
t1 = ax.text(x[tpos], f[tpos],  str(slider1.val))

tpos = int(0.75*x.shape[0])
t2 = ax.text(x[tpos], f[tpos],  str(slider1.val))

def update(val):
    f = slider1.val*np.sin(x)
    l.set_ydata(f)

    # update the value of the Text object
    tpos = int(0.25*x.shape[0])
    t1.set_position((x[tpos], f[tpos]))
    t1.set_text(str(slider1.val))

    tpos = int(0.75*x.shape[0])
    t2.set_position((x[tpos], f[tpos]))
    t2.set_text(str(slider1.val))

    plt.draw()

slider1.on_changed(update)
plt.show()

which looks like,

The alternative may be to clear and redraw the text each time but this is slower and will be more hassle.