Unhandled exception when setting log axis on negative values

[thomashampson]

Describe the bug When attempting to set an axis to log on a colourfill where the axis values are negtive, there is an unhandled exception:

Traceback (most recent call last):
  File "/opt/Mantid/lib/workbench/plotting/propertiesdialog.py", line 45, in on_ok
    self.canvas.draw()
  File "/usr/lib/python3/dist-packages/matplotlib/backends/backend_qt5agg.py", line 127, in draw
    super(FigureCanvasQTAggBase, self).draw()
  File "/usr/lib/python3/dist-packages/matplotlib/backends/backend_agg.py", line 430, in draw
    self.figure.draw(self.renderer)
  File "/usr/lib/python3/dist-packages/matplotlib/artist.py", line 55, in draw_wrapper
    return draw(artist, renderer, *args, **kwargs)
  File "/usr/lib/python3/dist-packages/matplotlib/figure.py", line 1299, in draw
    renderer, self, artists, self.suppressComposite)
  File "/usr/lib/python3/dist-packages/matplotlib/image.py", line 138, in _draw_list_compositing_images
    a.draw(renderer)
  File "/usr/lib/python3/dist-packages/matplotlib/artist.py", line 55, in draw_wrapper
    return draw(artist, renderer, *args, **kwargs)
  File "/usr/lib/python3/dist-packages/matplotlib/axes/_base.py", line 2437, in draw
    mimage._draw_list_compositing_images(renderer, self, artists)
  File "/usr/lib/python3/dist-packages/matplotlib/image.py", line 138, in _draw_list_compositing_images
    a.draw(renderer)
  File "/opt/Mantid/lib/mantid/plots/resampling_image/samplingimage.py", line 72, in draw
    self._resample_image()
  File "/opt/Mantid/lib/mantid/plots/resampling_image/samplingimage.py", line 107, in _resample_image
    xbins, ybins = self._calculate_bins_from_extent()
  File "/opt/Mantid/lib/mantid/plots/resampling_image/samplingimage.py", line 98, in _calculate_bins_from_extent
    xbins = int(np.ceil(bbox.width * dpi))
ValueError: cannot convert float NaN to integer

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/opt/Mantid/lib/workbench/plotting/propertiesdialog.py", line 49, in on_ok
    self.canvas.draw()
  File "/usr/lib/python3/dist-packages/matplotlib/backends/backend_qt5agg.py", line 127, in draw
    super(FigureCanvasQTAggBase, self).draw()
  File "/usr/lib/python3/dist-packages/matplotlib/backends/backend_agg.py", line 430, in draw
    self.figure.draw(self.renderer)
  File "/usr/lib/python3/dist-packages/matplotlib/artist.py", line 55, in draw_wrapper
    return draw(artist, renderer, *args, **kwargs)
  File "/usr/lib/python3/dist-packages/matplotlib/figure.py", line 1299, in draw
    renderer, self, artists, self.suppressComposite)
  File "/usr/lib/python3/dist-packages/matplotlib/image.py", line 138, in _draw_list_compositing_images
    a.draw(renderer)
  File "/usr/lib/python3/dist-packages/matplotlib/artist.py", line 55, in draw_wrapper
    return draw(artist, renderer, *args, **kwargs)
  File "/usr/lib/python3/dist-packages/matplotlib/axes/_base.py", line 2437, in draw
    mimage._draw_list_compositing_images(renderer, self, artists)
  File "/usr/lib/python3/dist-packages/matplotlib/image.py", line 138, in _draw_list_compositing_images
    a.draw(renderer)
  File "/opt/Mantid/lib/mantid/plots/resampling_image/samplingimage.py", line 72, in draw
    self._resample_image()
  File "/opt/Mantid/lib/mantid/plots/resampling_image/samplingimage.py", line 107, in _resample_image
    xbins, ybins = self._calculate_bins_from_extent()
  File "/opt/Mantid/lib/mantid/plots/resampling_image/samplingimage.py", line 98, in _calculate_bins_from_extent
    xbins = int(np.ceil(bbox.width * dpi))
ValueError: cannot convert float NaN to integer

To Reproduce

1. Load SANSLOQCan2D.xs from the training course data, but set SpectrumMin = 1 and SpectrumMax = 20
2. Right click and select "Plot>Colorfill"
3. Double click a value on the y-axis to open the "Edit axis" dialog.
4. Select the "Log" checkbox and click OK.

Expected behavior Either disable the log button or display a message when attempting to apply the option.

Platform/Version (please complete the following information):

• OS: Ubunutu
• OS Version: 18.04
• Confirmed in 6.2 and 6.3

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[thomashampson]

This no longer causes an exception, maybe matplotlib handles it better now.