function plot_interp(x,y,xd,yd,plottitle)

pp = spline(xd, yd);
yspline = ppval(pp, x);

pp  = pchip(xd, yd);
ypchip = ppval(pp, x);

pp = simple_cubic_setup(xd, yd);
ysimple = ppval(pp, x);

plot(x,y,'k-',  xd,yd,'k+', x,ysimple,'m-', x,yspline,'g-', x,ypchip,'b-');
legend('true','data','simple cubic','spline','pchip', 'location', 'southeast');
legend('boxon');
title(plottitle);

return