Catch outputs being the same as inputs for histogramming:

Traceback (most recent call last):
  File "veusz/qtwidgets/datasetbrowser.pyc", line 349, in refresh
  File "veusz/qtwidgets/datasetbrowser.pyc", line 280, in makeGrpTreeFilename
  File "veusz/qtwidgets/datasetbrowser.pyc", line 260, in makeGrpTree
  File "veusz/qtwidgets/datasetbrowser.pyc", line 58, in __init__
  File "veusz/document/datasets.pyc", line 596, in userSize
  File "veusz/document/dataset_histo.pyc", line 265, in <lambda>
  File "veusz/document/dataset_histo.pyc", line 253, in getData
  File "veusz/document/dataset_histo.pyc", line 141, in getBinVals
  File "veusz/document/dataset_histo.pyc", line 58, in getData
  File "veusz/document/dataset_histo.pyc", line 265, in <lambda>
  File "veusz/document/dataset_histo.pyc", line 253, in getData
  File "veusz/document/dataset_histo.pyc", line 141, in getBinVals
...