xc = -2:.2:2.
yc = xc + .1 * xc.^3
xm = -2:.04:2.;
%ym = spline(xc, yc, xm, 'spline');
ym = interp1(xc, yc, xm, 'spline');
figure
plot(xc,yc,'*',xm,ym,'-')
ymr = xm + .1 * xm.^3
diff =ym - ymr;
figure
plot(xm, 100.*diff./ymr)
figure
plot(xm, diff)