try something like this (it is very straight forward),
x=0:.1:10;
plot(x,yourfunctionname(x));
function [ y ] = yfunc( x )
%YFUNC Summary of this function goes here
% Detailed explanation goes here
for i=1:size(x,1)
if x(i,1) > 0 & x(i,1) <= 2
y(i,1)=2*x(i,1);
elseif x(i,1)>2 & x(i,1)<4...