private void updateHistogram(ArrayList<Chord> chords) {
		Chord prev, next;	
		for(int i=0; i<chords.size()-1; i++){
			prev = chords.get(i);
			next = chords.get(i+1);
			String previousChord = prev.toString();
			String nextChord = next.toString();
			if(!chordList.containsKey(previousChord)){
				chordList.put(previousChord, prev);
			}
			if(!chordList.containsKey(nextChord)){
				chordList.put(nextChord, next);
			}
			if(histogram.containsKey(previousChord)){
				if(histogram.get(previousChord).containsKey(nextChord)){
					int count = histogram.get(previousChord).get(nextChord);
					histogram.get(previousChord).put(nextChord, count+1);
				}else{
					histogram.get(previousChord).put(nextChord, 1);
				}
			}else{
				HashMap<String, Integer> notesAfter = new HashMap<String, Integer>();
				histogram.put(previousChord, notesAfter);
			}
		}
	}