Fix direction; add scale computing.

roentgen/util.py

@@ -1,8 +1,20 @@
 class MinMax:
+    """
+    Minimum and maximum.
+    """
     def __init__(self):
         self.min_ = None
         self.max_ = None
 
-    def add(self, value):
+    def update(self, value):
+        """
+        Update minimum and maximum with new value.
+        """
         self.min_ = value if not self.min_ or value < self.min_ else self.min_
         self.max_ = value if not self.max_ or value > self.max_ else self.max_
+
+    def delta(self):
+        """
+        Difference between maximum and minimum.
+        """
+        return self.max_ - self.min_