docs: def color_sequence(palette=None, n_colors=None):
      """
      Return a `ListedColormap` object from a named sequence palette. Useful
      for continuous color scheme values and color maps.
  
      Calling this function with ``palette=None`` will return the default
      color sequence: Color Brewer RdBu.
  
      Parameters
      ----------
  
      palette : None or str or sequence
          Name of a palette or ``None`` to return the default palette. If a
          sequence the input colors are used to create a ListedColormap.
  
          The currently implemented color sequences are from Color Brewer.
  
          Available palette names from :py:mod:`yellowbrick.colors.palettes` are:
  
          .. hlist::
              :columns: 3
  
              * :py:const: Blues
              * :py:const: BrBG
              * :py:const: BuGn
              * :py:const: BuPu
              * :py:const: GnBu
              * :py:const: Greens
              * :py:const: Greys
              * :py:const: OrRd
              * :py:const: Oranges
              * :py:const: PRGn
              * :py:const: PiYG
              * :py:const: PuBu
              * :py:const: PuBuGn
              * :py:const: PuOr
              * :py:const: PuRd
              * :py:const: Purples
              * :py:const: RdBu
              * :py:const: RdGy
              * :py:const: RdPu
              * :py:const: RdYlBu
              * :py:const: RdYlGn
              * :py:const: Reds
              * :py:const: Spectral
              * :py:const: YlGn
              * :py:const: YlGnBu
              * :py:const: YlOrBr
              * :py:const: YlOrRd
              * :py:const: ddl_heat
  
      n_colors : None or int
          Number of colors in the palette. If ``None``, the default will depend
          on how ``palette`` is specified - selecting the largest sequence for
          that palette name. Note that sequences have a minimum lenght of 3 - if
          a number of colors is specified that is not available for the sequence
          a ``ValueError`` is raised.
  
      Returns
      -------
      colormap
          Returns a ListedColormap object, an artist object from the matplotlib
          library that can be used wherever a colormap is necessary.
      """
      # Select the default colormap if None is passed in.
      palette = palette or DEFAULT_SEQUENCE
  
      # Create a listed color map from the sequence
      if not isinstance(palette, str):
          return mplcol.ListedColormap(palette)
  
      # Otherwise perform a case-insensitive lookup
      sequences = {key.lower(): key for key in SEQUENCES.keys()}
      if palette.lower() not in sequences:
          raise YellowbrickValueError(
              "'{}' is not a recognized palette!".format(palette)
          )
  
      # Collect the palette into the dictionary of lists.
      n_palettes = SEQUENCES[sequences[palette.lower()]]
  
      # If no definitive color is passed in, maximize it.
      if n_colors is None:
          n_colors = max(n_palettes.keys())
  
      else:
          if n_colors not in n_palettes.keys():
              raise YellowbrickValueError(
                  "No {} palette of length {}".format(palette, n_colors)
              )
  
      # Return the color map from the sequence
      return mplcol.ListedColormap(n_palettes[n_colors], name=palette, N=n_colors)