fix fft premature slicing away of 0 freq bin

also fixes:
- use rfft for better efficiency
- use rfftfreq to compute coords correctly
  - works for both odd/even lengths
- python3: integer division needed for numpy indexing

pyqtgraph/graphicsItems/PlotDataItem.py

@@ -679,10 +679,9 @@ class PlotDataItem(GraphicsObject):
             x2 = np.linspace(x[0], x[-1], len(x))
             y = np.interp(x2, x, y)
             x = x2
-        f = np.fft.fft(y) / len(y)
+        f = np.fft.rfft(y) / y.size
-        y = abs(f[1:len(f)/2])
+        x = np.fft.rfftfreq(y.size)
-        dt = x[-1] - x[0]
+        y = np.abs(f)
-        x = np.linspace(0, 0.5*len(x)/dt, len(y))
         return x, y
     
 def dataType(obj):