How to Interpolate a value in between valures

Hey there. I have this really simple problem which i cant seem to figure out. i have two vectors which i want to interpolate for the x values:

if true
 z=[166 173 182 195 216 252];
 x=[1.31 1.36 1.44 1.54 1.71 1.99] ;
 xi=linspace(1.31,1.99,100);
 zi=interp1(x,z,xi);
 finish=[xi;jo]'
end

how can i make sure that for xi it uses all my data for when it is not a linear function?