There are multiple polygons that could be drawn this way. You must describe some algorithm for how to order them, such as the angle to the next point must always be clockwise (or counterclockwise) or something like that. You could find the centroid (average of x and y) then calculate angles of all points, and then sort based on angles. Is that what you want to do?
Here's some untested code:
meanx = mean(x);
meany = mean(y);
angles = atan( (y-ymean) ./ (x-xmean));
[sortedAngles, sortIndices] = sort(angles);
reorderedx = x(sortIndices);
reorderedy = y(sortIndices);
plot(reorderedx, reorderedy, 'bo-');
