Update holoviews to 1.9.1

[pyup-bot]

There's a new version of holoviews available.
You are currently using 1.8.4. I have updated it to 1.9.1

These links might come in handy: PyPI | Changelog | Homepage

Changelog

1.9.1

=============

This release is a minor bug fix release patching various issues
which were found in the 1.9.0 release.

Enhancements:

• Exposed min_alpha parameter on datashader shade and datashade
  operations (\2109)

Fixes:

• Fixed broken bokeh server linked stream throttling
  (\2112)
• Fixed bug in bokeh callbacks preventing linked streams using
  bokeh's on_event callbacks from working
  (\2112)
• Fixed insufficient validation issue for Image and bugs when
  applying regrid operation to xarray based Images
  (\2117)
• Fixed handling of dimensions and empty elements in univariate_kde
  and bivariate_kde operations
  (\2103)

1.9.0

=============

This release includes a large number of long awaited features,
improvements and bug fixes, including streaming and graph support,
binary transfer of bokeh data, fast Image/RGB regridding, first-class
statistics elements and a complete overhaul of the geometry elements.

Particular thanks to all users and contributers who have reported
issues and submitted pull requests.

Features:

• The kdim and vdim keyword arguments are now positional making the
  declaration of elements less verbose (e.g. Scatter(data, 'x',
  'y')) (\1946)
• Added Graph, Nodes, and EdgePaths elements adding support for
  plotting network graphs
  (\1829)
• Added datashader based regrid operation for fast Image and RGB
  regridding (\1773)
• Added support for binary transport when plotting with bokeh,
  providing huge speedups for dynamic plots
  (\1894,
  \1896)
• Added Pipe and Buffer streams for streaming data support
  (\2011)
• Add support for datetime axes on Image, RGB and when applying
  datashading and regridding operations
  (\2023)
• Added Distribution and Bivariate as first class elements which can
  be plotted with matplotlib and bokeh without depending on seaborn
  (\1985)
• Completely overhauled support for plotting geometries with Path,
  Contours and Polygons elements including support for coloring
  individual segments and paths by value
  (\1991)

Enhancements:

• Add support for adjoining all elements on matplotlib plots
  (\1033)
• Improved exception handling for data interfaces
  (\2041)
• Add groupby argument to histogram operation
  (\1725)
• Add support for reverse sort on Dataset elements
  (\1843)
• Added support for invert_x/yaxis on all elements
  (\1872,
  \1919)

Fixes:

• Fixed a bug in matplotlib causing the first frame in gif and mp4
  getting stuck
  (\1922)
• Fixed various issues with support for new nested categorical axes
  in bokeh (\1933)
• A large range of other bug fixes too long to list here.

Changes affecting backwards compatibility:

• The contours operation no longer overlays the contours on top of
  the supplied Image by default and returns a single
  Contours/Polygons rather than an NdOverlay of them
  (\1991)
• The values of the Distribution element should now be defined as a
  key dimension
  (\1985)
• The seaborn interface was removed in its entirety being replaced
  by first class support for statistics elements such as
  Distribution and Bivariate
  (\1985)
• Since kdims and vdims can now be passed as positional arguments
  the bounds argument on Image is no longer positional
  (\1946).
• The datashade and shade cmap was reverted back to blue due to issues
  with the fire cmap against a white background.
  (\2078)
• Dropped all support for bokeh versions older than 0.12.10
• histogram operation now returns Histogram elements with less
  generic value dimension and customizable label
  (\1836)

Got merge conflicts? Close this PR and delete the branch. I'll create a new PR for you.

Happy merging! 🤖

[pyup-bot]

Closing this in favor of #139