function plot_parabola(xi, yi)
xdata = linspace(xi(1), xi(end));
ydata = polyval( polyfit(xi, yi, 2), xdata);
hold on
plot(xi, yi, 'o', xdata, ydata)
plot([xi( 1 ) xi( 1 )], [0 yi( 1 )]);
plot([xi(end) xi(end)], [0 yi(end)]);
hold off