plot difference between graphs

i have made the following code but do not know how to plot the difference between the two graphs, the code as it is now, plots the two lines on the same graph, but i want to plot the differnece between these two graphs and do not know how to do so. the data to be graphed needs to still ahve all the zeros removed

%position

    file_name = 'GenLowpass8hz.data';
    data = dlmread(file_name,'\t',0,0);

    %coulmns
   column_1=data(:,1); %  Chan1 Biceps Raw  
     column_2=data(:,2); %   Chan2 Wrist Extensor Raw                      
     column_3=data(:,3); %  Chan3 Filtered EMG 
     column_4=data(:,4);   %    Chan4 Ball Position (radians)                  
     column_5=data(:,5);  %   Chan5 Elbow Position (rad)

     %creating time vector

DataSize=size(data,1);

t= 0:(1/960): ((DataSize-1)/960);

%plot data

%remove zeros

index=find(data(:,4)>0);

idx=find(data(:,5)>0);

subplot(3,2,1)

plot(t(index),column_4(index),':ob')

xlim([0 20])

ylim([6 9.5])

hold on

plot(t(idx), column_5(idx),':xr')

hold off

legend('ball','elbow')

   title('LowPass 8Hz', 'FontSize',18);

      xlabel('Time (s)');

      ylabel('Position (rad)');

ignore the attached image

[Walter Roberson]

What do you want to have happen at the t locations for which one of the two has a value but the other does not? If you want to interpolate between adjacent values then you will need to indicate which kind of interpolation to use.