Unable to plot graph on matlab using arduino

Hi all, I am new to Arduino so please help me on this. For my school project, I am required to log data produced by my Arduino using matlab on realtime. Here is my code for Matlab: By using this code I am supposed to get 4 outputs on my Arduino that I can use to plot my values to time graph using matlab. Hence I have written a code for matlab (for 2 of the sensors as an initial test):

clear all
a = arduino ;
line1 = line(nan, nan, 'color', 'red');
line2 = line(nan, nan, 'color', 'blue');
i = 0;
while 1
  sensor1 = readDigitalPin(a, 'D3');
  sensor2 = readDigitalPin(a, 'D6');
  pause(0.5);

    x1= get(line1, 'xData');
    y1= get(line1, 'yData');
    x2= get(line2, 'xData');
    y2= get(line2, 'yData');

    x1=[x1 i];
    y1=[y1 sensor1];
    x2=[x2 i];
    y2=[y2 sensor2];

    set(line1, 'xData', x1, 'yData', y1);
    set(line2, 'xData', x2, 'yData', y2);

    i=i+1;
    pause(1)
end
However with that I am unable to get an output graph at all and I am unable to understand why. Is there any problems with my above code?