`python manual_mask.py` in xray-vision

gistfile1.txt

after pressing 'l' and hovering over the image...
```
Traceback (most recent call last):
  File "/home/eric/anaconda/envs/nikea3/lib/python3.4/site-packages/matplotlib/backends/backend_qt5.py", line 282, in mouseMoveEvent
    FigureCanvasBase.motion_notify_event(self, x, y)
  File "/home/eric/anaconda/envs/nikea3/lib/python3.4/site-packages/matplotlib/backend_bases.py", line 1935, in motion_notify_event
    guiEvent=guiEvent)
  File "/home/eric/anaconda/envs/nikea3/lib/python3.4/site-packages/matplotlib/backend_bases.py", line 1527, in __init__
    LocationEvent.__init__(self, name, canvas, x, y, guiEvent=guiEvent)
  File "/home/eric/anaconda/envs/nikea3/lib/python3.4/site-packages/matplotlib/backend_bases.py", line 1444, in __init__
    trans = self.inaxes.transData.inverted()
  File "/home/eric/anaconda/envs/nikea3/lib/python3.4/site-packages/matplotlib/transforms.py", line 2323, in inverted
    return CompositeGenericTransform(self._b.inverted(), self._a.inverted())
  File "/home/eric/anaconda/envs/nikea3/lib/python3.4/site-packages/matplotlib/transforms.py", line 2323, in inverted
    return CompositeGenericTransform(self._b.inverted(), self._a.inverted())
  File "/home/eric/anaconda/envs/nikea3/lib/python3.4/site-packages/matplotlib/transforms.py", line 1728, in inverted
    self._inverted = Affine2D(inv(mtx), shorthand_name=shorthand_name)
  File "/home/eric/anaconda/envs/nikea3/lib/python3.4/site-packages/numpy/linalg/linalg.py", line 520, in inv
    ainv = _umath_linalg.inv(a, signature=signature, extobj=extobj)
  File "/home/eric/anaconda/envs/nikea3/lib/python3.4/site-packages/numpy/linalg/linalg.py", line 90, in _raise_linalgerror_singular
    raise LinAlgError("Singular matrix")
numpy.linalg.linalg.LinAlgError: Singular matrix

```