scales 1.4.0

• Range training now supports custom classes. Continuous classes require a
  range() method that returns numeric values. Discrete classes require a
  levels() method (and optionally a droplevels() method) (#480).
• New label_glue() labelling function for interpolated strings (#457).
• fullseq() and by extension breaks_width() can now deal with unsorted
  ranges (#435).
• New label_date_short(leading) argument to replace leading zeroes (#442)
• breaks_pretty() will return the input limit when it has no range (#446)
• breaks_exp() now provides more sensible breaks for the exponential transform
• transform_exp() now has more sensible breaks, available in breaks_exp()
  (@teunbrand, #405).
• The scales package now keeps track of known palettes. These can be retrieved
  using get_palette() or registered using set_palette() (#396).
• label_log() has a signed argument for displaying negative numbers
  (@teunbrand, #421).
• New label_dictionary() for named lookup of labels (#458).
• New function compose_label() to chain together label formatting functions
  (#462)
• Palette function now have classes, attributes and coercion methods
  (@teunbrand, #426).
• Added colour manipulation functions: col_shift(), col_saturate(),
  col_darker(), col_lighter() and col_mix() (@teunbrand, #423)
• label_date_short() gains tz and locale argument (#478)