x=[ 1  3   8   6  12  17 ];
y=[ 4  6  -5   0  -9  -7 ];

M = 100;

%itype = 'cubic';
itype = 'spline';

xi = linspace(min(x), max(x), M);
yi = interp1(x, y, xi, itype);

xe = linspace(0, 20, M);
ye = interp1(x, y, xe, itype, 'extrap');


h=plot(x,y,'o', xe, ye, xi, yi );
ylabel('y')
xlabel('x')
set(h,'markersize',2.5);
grid on
legend('data points','extrapolation','interploation')