Question

I used sliders in matplotlib to update a few graphs based on GUI input.
All my graphs update well.
But when I use figtext, I have the problem that the updated text will write over the existing one.

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

p.subplot(111) 
x = np.arange(0,500,1)
f = np.sin(x/100.0)
l11, = p.plot(f)

ax = p.axes([0.25, 0.05, 0.7, 0.03], axisbg='lightgoldenrodyellow')
slider1 = Slider(ax, 'amplitude', -1.0, 1.5, valinit=0)

def update(val):
    f = slider1.val * np.sin(x/100.0)
    l11.set_ydata(f)
    np.set_printoptions(precision=2)
    p.figtext(0.5, 0.65,  str(slider1.val) )
    p.draw()

slider1.on_changed(update)
p.show()

Answer 1

Every time you call p.figtext(0.5, 0.65, str(slider1.val)) you are creating a new Text object which is being written on top of the previous ones. What you should do is save a reference to the first Text object and update its contents by calling its set_text() method. I have updated your code with a working example.

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

p.subplot(111) 
x = np.arange(0,500,1)
f = np.sin(x/100.0)
l11, = p.plot(f)

ax = p.axes([0.25, 0.05, 0.7, 0.03], axisbg='lightgoldenrodyellow')
slider1 = Slider(ax, 'amplitude', -1.0, 1.5, valinit=0)

# the text on the figure
fig_text = p.figtext(0.5, 0.65,  str(slider1.val))

def update(val):
    f = slider1.val*np.sin(x/100.0)
    l11.set_ydata(f)
    np.set_printoptions(precision=2)

    # update the value of the Text object
    fig_text.set_text(str(slider1.val))

    p.draw()

slider1.on_changed(update)
p.show()