Matplotlib: contour plot with slider widget

Question

Newbie matplotlib user here. I'm trying to use a slider to adjust a parameter in a contour plot, but when I do so, I get:

AttributeError: QuadContourSet instance has no attribute 'set_data'

I suspect that I'm calling set_data on the wrong object, but I can't find any documentation on what the right object is. Can you help? Thanks.

Here's the full code:

import numpy as np
import matplotlib as mpl
import matplotlib.mlab as mlab
import matplotlib.pyplot as pyl
from matplotlib.contour import QuadContourSet
from matplotlib.widgets import Slider

#Define display parameters
mpl.rcParams['xtick.direction'] = 'out'
mpl.rcParams['ytick.direction'] = 'out'
delta = 0.025

#Define model parameters
alpha = .5
beta = .5
x_bar, a, b, c = 2, 0, 1, .1
v = np.arange(0, 10, delta)
w = np.arange(0, 10, delta)

#Calculate grid values
V, W = np.meshgrid(v,w)
Z = (V**(beta))*(W**(1-beta))
X = x_bar + a + b*Z
U = alpha*np.log(V) + (1-alpha)*np.log(X) - c*(W+V)

# Plot
fig = pyl.figure()

ax = fig.add_subplot(221)
CS = QuadContourSet(pyl.gca(), V, W, U, 200)
pyl.clabel(CS, inline=1, fontsize=10)
pyl.title('Simplest default with labels')

#Define slider for alpha
axcolor = 'lightgoldenrodyellow'
alpha_axis  = pyl.axes([0.25, 0.15, 0.65, 0.03], axisbg=axcolor)
alpha_slider = Slider(alpha_axis, 'Amp', 0, 1, valinit=.5)

def update(val):
    alpha = alpha_slider.val
    U = alpha*np.log(V) + (1-alpha)*np.log(X) - c*(W+V)
    CS.set_data(V, W, U)
    pyl.draw()

alpha_slider.on_changed(update)

pyl.show()

Answer 1

The problem is that the QuadContourSet object has no way to update its data, since if you change the data arbitrarily, the whole thing needs to be recomputed. I don't know if there is something about your particular way of generating data that would lend itself to a simpler way to modify the contour lines, but if not, I think what you need to do is to plot the contours from scratch:

# After your "Define model parameters" block

def compute_and_plot(ax, alpha):
    #Calculate grid values
    V, W = np.meshgrid(v,w)
    Z = (V**(beta))*(W**(1-beta))
    X = x_bar + a + b*Z
    U = alpha*np.log(V) + (1-alpha)*np.log(X) - c*(W+V)

    CS = QuadContourSet(ax, V, W, U, 200)
    pyl.clabel(CS, inline=1, fontsize=10)

# Plot
fig = pyl.figure()
pyl.title('Simplest default with labels')
ax = fig.add_subplot(221)
compute_and_plot(ax, alpha)

#Define slider for alpha
axcolor = 'lightgoldenrodyellow'
alpha_axis  = pyl.axes([0.25, 0.15, 0.65, 0.03], axisbg=axcolor)
alpha_slider = Slider(alpha_axis, 'Amp', 0, 1, valinit=.5)

def update(ax, val):
    alpha = alpha_slider.val
    ax.cla()
    compute_and_plot(ax, alpha)
    pyl.draw()

alpha_slider.on_changed(lambda val: update(ax, val))

pyl.show()