X = [0 1 3 4];
Y = [0 0 2 2];

figure(1);

hold on
scatter(X, Y,'filled')
hold off

x = linspace(X(1), X(end), 200);
y = ppval(spline(X,Y),x);

hold on
plot(x, y);
hold off

figure(2);

hold on
scatter(X, Y,'filled')
hold off

x = linspace(X(1), X(end), 200);
y = ppval(spline(X, [0 Y 0]),x);

hold on
plot(x, y);
hold off