Avoid error when computing ticks

When the axis is so small that no tick can be drawn, an exception used to be raised

Author: PierreRaybaut

qwt/scale_engine.py

@@ -561,6 +561,8 @@ def buildMajorTicks(self, interval, stepSize):
         :return: Calculated ticks
         """
         numTicks = min([round(interval.width()/stepSize)+1, 10000])
+        if np.isnan(numTicks):
+            numTicks = 0
         ticks = [interval.minValue()]
         for i in range(1, int(numTicks-1)):
             ticks += [interval.minValue()+i*stepSize]