x_data = [-1 -0.75 -0.5 -0.25 0 0.25 0.5 0.75 1];
y_data = [0.3333 0.4324 0.5714 0.7619 1.0000 1.2308 1.3333 1.2308 1.0000];
pp = spline(x_data, y_data);
x_plot=linspace(-1, 1, 50);
y_plot = ppval(pp, x_plot);
plot(x_data, y_data, 'o', x_plot, y_plot, '-b')