Adding large data sets within a for loop in matlab

I'm attempting to make a very basic gating system, however the smallest data I have is single cycle which equals a 18*100 array. I've attempted just plotting it with a hold on/off function and collecting the data after with a 'h=findobj(gca,'Type','line');'. However this takes forever and requires a lot of reshaping. Is there a simpler way to either store the data or add the complete arrays (not sum as that does line by line which is a no-no) together in the for loop?

h=findobj(gca,'Type','line'); %data retrieved from orginal figure
x=get(h,'Xdata');
y=get(h,'Ydata');
X=reshape(x,(18),[]);
Y=reshape(y,(18),[]);

hold on
    for i=1:4;
     xx=X(:,i);
     yy=Y(:,i);
     gx=cell2mat(xx); 
     gy=cell2mat(yy); 
    plot(gx) % manipulated data from orginal figure, 
    plot(gy) % plot required to extract all the for loop data
    end
hold off

Basically I just want to add the four gx together and divide them, however they have to be added as bulk, not line by line as one cycle of the loop equals a cycle of the system. (Also the 4 is just a number it really is more like 60+, which is why I can't just do it manually).

Many thanks!