Plotting error bars on a smoothed spline using plotyy

I'm using plotyy to plot two functions on different axes that are the smoothing of two data sets.

xx = 1:.1:12;  
yyCLAMP = spline(x,y,xx);
yyMSL2 = spline(x,y,xx);

[AX2,CLAMPtrend,MSL2trend]=plotyy(xx,yyCLAMP,xx,yyMSL2);

I need to add error bars to these curves from the data set that I used to create them. I already have my error values calculated from a separate algorithm. I have them in vectors E1 and E2.

I've been trying to plot them using hold:

hold (AX2(1))
errorbar(x,y,E,'x');

hold (AX2(2))
errorbar(x,y,E2,'x');

When I run this though, the first set of error bars gets applied perfectly. But the second set is not. They are plotted, but seem to be vertically offset by a significant amount.

Any ideas of the cause?