The MATLAB function errorbar allows you to separately define the lengths of the upper and lower errors. So, you can set the lower value to be zero.
For example
x = 1:5
y = 2:6;
u = [1 2 1 2 1];
l = zeros(1,5);
figure
errorbar(x,y,l,u)
ylim([0 8])
The herrorbar function ( http://www.mathworks.com/matlabcentral/fileexchange/3963-herrorbar ) that I told you about in an earlier thread similarly allows you to separately define the left and right lengths.
