How to create a line passing through the inner boundary of the point set shown in the image?

I have several points with (X,Z) coordinates in a txt file. I have read all the point into column matrix X and Z. Then I plotted a 2D scatter plot. The blue dots are the points with Xand Z coordinates. I want to create a line (which is shown in red) passing through the inner boundary of points?

I tried the follwing code. the result shown below is not what I'm expecting. I'm really greatful for anyone who helps me solve the problem.

scatter(X,Z,1);

k= boundary (X,Z) ;

hold on;

plot (X(k), Z(k));