The problem is that MATLAB doesn't know what curve you want, since there is no data between the points you have. You either need more data resolution, or you can tell MATLAB to make a smooth curve by interpolating with a spline:
xNew = 0:0.1:160;
yNew = interp1(x,y,xNew,'spline');
However, remember that the smooth curve is made up by MATLAB, so it could be misleading to show smooth curves if there are not really any in reality.