Why aren't there lines between data points in Contourf plot?

Question

I plotted a contourf plot with 9x11 points. When I plot the contourf plot then I expect to see lines between data points (since there is no other data in between the data points). But for example in the 0.9 level there are some parts (especially between x=2 and x=4) which are not linear. What can be the reason for that?

plt.figure()
x=np.linspace(0,10,11)
y=np.linspace(0,10,11)
X,Y = np.meshgrid(x,y)
levels = np.arange(0,1.01,0.1)
norm = cm.colors.Normalize(vmax=1, vmin=0)
cmap = cm.PRGn

CS1 = plt.contourf(X, Y, data,levels=levels,cmap=cm.get_cmap(cmap, 
        len(levels) - 1),norm=norm)
plt.xticks(np.arange(11),np.arange(11))
plt.yticks(np.arange(11),np.arange(250,855,55))
plt.xlim([0,8])
plt.colorbar(CS1)
plt.grid()
plt.show()

Answer 1

It's supposed to be that way: contourf colours in the area between the lines and contour draws the lines. See the examples.

Answer 2

Maybe the following plot helps to understand a contour plot better.

Here we plot a contour of an array with 3x3 points. The value of the middle point (6) is much larger than the other values. We chose levels of 3 and 5 where to plot the contour lines. Those lines are calculated by the contour as to interpolate the data.
Using more points will then allow to use more lines and make them look more smooth.

import matplotlib.pyplot as plt
import numpy as np

X,Y = np.meshgrid(np.arange(3), np.arange(3))
Z = np.array([[1,1,1],[2,6,2],[1,1,1]])

fig, ax=plt.subplots()
cs = ax.contour(X,Y,Z, levels=[3,5])
cs2 = ax.contourf(X,Y,Z, levels=[1,3,5,6], alpha=0.3)
plt.clabel(cs, cs.levels, inline=False)
plt.colorbar(cs2)
ax.scatter(X,Y)
for x,y,z in zip(X.flatten(), Y.flatten(), Z.flatten()):
    ax.text(x,y,z)

plt.show()