Fixed typo in qwt/interval.py: invalid intervals were having numpy.nan width

Author: PierreRaybaut

qwt/interval.py

@@ -54,7 +54,7 @@ def isValid(self):
             return self.__minValue < self.__maxValue
 
     def width(self):
-        if self.isValid:
+        if self.isValid():
             return self.__maxValue - self.__minValue
         else:
             return 0.