diff --git a/morris.coffee b/morris.coffee
index 206c188..40a847e 100644
--- a/morris.coffee
+++ b/morris.coffee
@@ -296,10 +296,13 @@ class Morris.Line
       year = parseInt(p[1], 10);
       y1 = new Date(year, 0, 1);
       y2 = new Date(year+1, 0, 1);
+      # first thursday in year (ISO 8601 standard)
       if y1.getDay() isnt 4
         y1.setMonth(0, 1 + ((4 - y1.getDay()) + 7) % 7);
+      # first thursday in following year
       if y2.getDay() isnt 4
         y2.setMonth(0, 1 + ((4 - y2.getDay()) + 7) % 7);
+      # Number of weeks between thursdays
       weeks = 1 + Math.ceil((y2 - y1) / 604800000);
       parseInt(p[1], 10) + (parseInt(p[2], 10) - 1) / weeks;
     else if n