PLot two cluster plot

I want to get a cluster plot as I would have 2 set of values One set which falls as positive set i.e y1 and the other set of values which fall in negative set i.e y2

My code:
> y1 = [11.0 10.8 11.0 11.10 11.9 9.9] ;
>> y2 = [14.0; 16.8; 18.0; 19.10; 21.9; 29.9] ;
>> figure
>> plot(y1,'ok','MarkerFaceColor','k')
>> hold on
>> plot(y2,'ok','MarkerFaceColor','r')

This works and i get a plot like this fig attached

However i am looking for something like getting two clusters against two values on x axis ie y1 and y2

and so i define the names for y1 and y2 and I try to plot them

names = {'Postive =y1' 'Negative =y2'};
 plot(names,y1,'ok','MarkerFaceColor','k')
hold on
set(gca,'xtick',[1:2],'xticklabel',names)
 plot(names,y1,'ok','MarkerFaceColor','k')

I still get numbers on my x axis whereas I want names on x axis