The plot you provided has 
on the horizontal axis. Therefore define x, for use as the horizontal coordinate for plotting: 
. Then 
. Therefore
on the horizontal axis. Therefore define x, for use as the horizontal coordinate for plotting: . Then . Therefore
.
x=-10:40;
M=[1,2,3,4,10,20];
Pout=zeros(length(x),length(M));
for i=1:length(M)
Pout(:,i)=(1-exp(-1./(10.^(x/10)))).^M(i);
end
semilogy(x,Pout(:,1),'-r',x,Pout(:,2),'-g',x,Pout(:,3),'-b',...
x,Pout(:,4),'-c',x,Pout(:,5),'-m',x,Pout(:,6),'-y');
ylabel('Pout'); grid on;
xlabel('10log_{10}(\gamma_{bar}/\gamma_0)');
ylim([1e-4,1]);
legend('M=1','M=2','M=3','M=4','M=10','M=20')
The plot above matches the plot you provided.
