How to reorder groups and corresponding curves in scatterhist

I have the following script aiming to show my data by scatterhist, properly. But it gives an awkward y-histogram, as it seems in the 'scatterhist' the curve correspond to the least frequent occurrences is plotted at the end; I mean in order. So, curves corresponding to more frequent occurrences are suppressed and do not illustrated properly. How can I fix my code.

p= 1:100; 
z= zeros(1,2);
n= -1:-1:-40;
a = [z,p,n];
c={zeros(numel(a)),1};
for i= 1:numel(a)
    if a(i)>0
        c(i)={'p'};
    elseif a(i)==0
        c(i)= {'z'};
    elseif a(i)<0
        c(i)={'n'};
    end
end
figure(1)
scatterhist(rand(1,numel(a)),a,'Group',char(c),'Marker','s..')