Help me plotting contour using contourf on 3 columns of data

Please help, i have 3 columns of data that consist of x(horizontal position), y(vertical position), and z (value at x,y position).

i want to make a contour plot like this: http://flic.kr/p/bqR4fT

but what i kept getting is random colour of lines like this http://flic.kr/p/bqR4ee

im already doing the

[XI,YI]=meshgrid(x,y) ZI=griddata(x,y,z,XI,YI,'nearest') %nearest neighbour contourf(XI,YI,ZI)

what did i do wrong?

thank you anyway, regards