step responce using lsim command making extra plot .

I want to make step responce using lsim command ( I know that there is a command names step but I need to do it using lsim command) . so my step is zero while 0<t<10 and is equel to 22 while 10<t<20 . when I use the command lsim . the plot is right but for some reason the output of the plot is giving me extra plot and I dont undestand why does it happen . is it a bug ? or am I using the command lsim wrong ?

clc;clear all;
t1=0:9
t2=10:20;
t=[t1 t2];
y1=0*ones(1,length(t1));
y2=1*ones(1,length(t2));
y3=[y1 y2]; %% step function that starts from 10 ;
sys1 = tf(1,[ 1 0]);%% integrator
lsim(sys1,y3,t); %% responce of step