This is one way. You can use meshgrid to turn vectors into matrices with values repeated in rows and/or columns. However, only Z must be a matrix. X and Y can be vectors. So here I define the slope along X, and then use meshgrid to repeat it for all values of Y. I tried to be tricky with computing the slope but mostly succeeded in making it look more complicated than it needs to be.
x = linspace(-7,9,17);
y = linspace(-4,4,9);
low = 2.6;
high = 3.4;
step = 0.1;
flat = 4;
z = (low-flat*step):step:(high+flat*step);
z(z<low) = low;
z(z>high) = high;
[Z,~] = meshgrid(z,y);
mesh(x,y,Z,'EdgeColor','k')
grid off
