How to compare parts of an Array

Hallo. I am new in programming and try to learn it. I wrote a Programm for analyzing an Hallsensor with an myDAQ. That is the Code:

    s = daq.createSession('ni'); 
addAnalogInputChannel(s,'myDAQ1', 0, 'Voltage'); 
addAnalogInputChannel(s,'myDAQ1', 1, 'Voltage'); 
s.Rate = 1000; % Scans pro Sekunde
s.IsContinuous = true; lh = addlistener(s,'DataAvailable',@plotData); 
startBackground(s); 
s.wait();

end

and the function plotData:

 function plotData(src, event)

    a = false;
    if ~a 
        A = event.Data;
        a = true;
    end
    %erster Array wird abgespeichert

    disp(event.Data)
    plot(event.TimeStamps, event.Data);
end

I wants to Programm an Jostick, which is usuing an hallsensor. With the Code and myDAQ I can Display the valtage Graphs for x and y axis. I wntas now to create a function that would Display the Position of the joistick as a dot but dont how to realize it. I thought of to save the first value and compare it with the other values during the session. Not sure how it to do.

Can somebody help me pls?