load fort.11 % the content is saved in a matrix called fort, see help load
x=fort;    
load fort.12 % this overwrites fort, so you must save it as x in previous step
y=fort;
figure(12)     % this will pop up another window called figure 12
plot(x,y,'x')

load fort.13
x1=fort(:,1); % x1 will then be the first column of fort
x2=fort(:,2); % x2 will then be the second column of fort
figure(11)     % this will pop up another window called figure 11
plot(x1,x2)

% alternatively, you can do
%% plot(fort(:,1),fort(:,2))