How can i reverse my plot?

Hello everyone, i am trying to code stream function in Matlab. i am done with alghoritmic part but having a problem with my contour plot. If you check the result for "u_new" matrix, it starts from 100 at the bottom and go to 0 when we move to upper parts. When i plot it with counter function, plot starts 100 value from top completely symmetric to what i want to plot? what is my mistake here? And also i want to limit x axis with 6 and y axis with 4 on my plot. Right now the interval is huge. Can you help me please. Here is my code;

clear all;
% for the task in CFD tutorial Video 12 - Matlab version of given C code
length_x = 6.0; % Length of x axis of confined space. 
length_y = 4.0;  
X = 31; % Number of points will be generated on X axis
Y = 21;
dx = length_x / (X-1);
dy = length_y / (Y-1);
criterion = 1e-8;

u_new=zeros(Y,X); %generating arrays to store the potential value (psi) of each point
u_old=zeros(Y,X);

% Defining coefficients of the discretized equation
a_p = 2 * (1 / dx^2 + 1 / dy^2);
a_e = 1 / (dx^2);
a_w = 1 / (dx^2);
a_n = 1 / (dy^2);
a_s = 1 / (dy^2);

% Defining Dirichlet Boundary Conditions
u_new(21 , 1/dx+2 : 1 : 6/dx+1) = 100;
u_old = u_new;

error = 1;
while error > criterion 
    error=0;
    % Defining Jacobi Iteration Method
    for i=2 : 1 : Y-1
        for j=2 : 1 : X-1
        u_new(i,j) = (1 / a_p) * (a_e * u_old(i+1,j) + a_w * u_old(i-1,j) + a_n * u_old(i,j+1) + a_s * u_old(i,j-1));
        end
    end

% Applying given Neumann Boundary Conditions
    for i=1 : 1 : Y
    u_new(i,X) = u_new(i,X-1);
    end
    
    
    error = 0 %Now we add the error term so in each general loop we will calculate the error and stop iteration when the amount go lower than the error we want to see
    for i=1 : 1 : Y
        for j=1 : 1 : X
            error = error + (u_new(i,j) - u_old(i,j))^2;
        end
    end
    error = sqrt(error / ((X-2) * (Y-2)));
    
   u_old = u_new; 
end

contour(u_new,'ShowText','on')

Answer 1

[Adam Danz]

I'm guessing you want to flip the y-axes.

set(gca,'YDir','reverse')