A common task is a graphical representation of a scalar function over a rectangle:
For this, first we have to generate a grid on the rectangle [a,b] x [c,d]. This is done using the meshgrid command:
n = ... # number of discretization points along the x-axis m = ... # number of discretization points along the x-axis X,Y = meshgrid(linspace(a,b,n), linspace(c,d,m))
X and Y are arrays with (n,m) shape such that 
contains the coordinates of the grid point 
as shown in the next figure (Figure 6.6):
Figure 6.6: A rectangle discretized by meshgrid
A rectangle discretized by meshgrid will be used to visualize the behavior of an iteration. Bur first we will use it to plot level curves of a function. This is done by the command contour.
As an example we choose Rosenbrock's banana function:
It is used to challenge optimization methods. The function values descend towards a banana-shaped valley, which itself decreases slowly towards the function’s global minimum at...
