The q variables are logical vectors indicating whether each angle is in a particular quadrant. data(q3==1,1) selects only the rows of data that are in quadrant 3, and selects column 1 from the resulting array.
Instead of modifying the existing code to change the logic, go back to the original logic, but copy the assignment of those constants to data into a new variable planar_data, then comment out the assignment to data; then in the planar_data array, set the third column to your sample compass degrees that you want for testing purposes. After that step, assign
data = [planar_data(:,1,2) mod(90 -planar_data(:,3),360)];
Once you have the demonstration going, make it in to a function that takes planar_data as a parameter instead of assigning hard-coded values to it.