v2.8.0

Added

• Introduce horizontal colorbars [#6024]
• Implement legend.grouptitlefont and hoverlabel.grouptitlefont [#6040]
• Add texttemplate and textfont to heatmap and histogram2d traces as well as
  histogram2dcontour and contour traces when coloring is set "heatmap" [#6028]

Fixed

• Fix to discard negative values from pie chart post-aggregation instead of during summation [#6051],
  with thanks to @destiny-wu for the contribution!