Well, I will answer it myself, 1. Do not use hold; 2. Put all the lines together;
So the final code will be something like,
hAx=plotyy([x1;x2;x3;....;xn],[y1;y2;y3;...;yn],[xa;xb;xc;...;xz],[ya;yb;yc;...;yz]);
x1~xn and y1~yn should have the same length; xa~xz and ya~yz should have the same length;
If the length is differnt, use interpolation to make them with the same length.