By default the categories are sorted into character order, not alphanumeric order.
S = "M"+(1:10);
A = categorical(S)
C = categories(A) % default order = sorted by character.
Y = 1:10;
plot(A,Y);
Solution one: You can specify the order when creating the categorical array:
D = categorical(S, "M"+(1:10)) % looks the same as A ...
categories(D) % but the categories are in the expected order.
Solution two: change the category order of an existing categorical array using REORDERCATS, e.g.:
[~,X] = sort(str2double(replace(C,"M","")));
B = reordercats(A,C(X));
categories(B)
E = reordercats(A,natsort(categories(A)));
categories(E)
Once you have the categories in the required order, then your plot will look as you expect:
plot(E,Y);
