While it is not strictly necessary, I think this would be conceptually related to moving bar() to return a single PathCollection (#22976), which is a fair bit of work but which I now think could be done in a way similar to the ContourSet transition recently merged (#25247).

Gradient bars

Note that #23265 is asking for "draw 3d bars gradient", which is the second example in https://de.mathworks.com/help/matlab/creating_plots/color-3-d-bars-by-height-1.html.

Even with a scalarmappable bar, we'll not be able to support this. One could achive this with a value-scaled variation of https://matplotlib.org/stable/gallery/lines_bars_and_markers/gradient_bar.html#sphx-glr-gallery-lines-bars-and-markers-gradient-bar-py. But I think we're far from internal code structures or an API needed to include this as a builtin feature.

For now, I'd say, this are out of scope.

Color-coded bars

This is fundamentally reasonable, even though #26310 is not making a good case for it.

We have basically two cases:

1. discrete colors that show up in the legend.

Examples:

• https://vizzlo.com/create/color-coded-bar-chart
• https://community.plotly.com/t/colorscale-in-bar-chart/15509

These are already handled via https://matplotlib.org/stable/gallery/lines_bars_and_markers/bar_colors.html

2. Continuous colors.

I've found two classes of examples:

• Purely artistic ones
  
  These do not need a legend and thus,

  colors = plt.cm.viridis(np.linspace(0, 1, len(labels)))
  plt.bar(labels, values, colors=colors)
  

  is enough. (Apart from the question whether one should do this at all).

• Continous color coding:
  e.g.
  

This needs a colorbar as reference and is the only case possibly warranting an extension of our API. Be aware though that bars have color, edgecolor and ecolor (errorbars). And that we need a conscious decision how to handle them (likely only provide colormapping for the face color).

I think this would be conceptually related to moving bar() to return a single PathCollection

rather replace BarContainer.patches (currently list of Rectangle) by a collection. (t.b.d. wether a general PathCollection is suitable or whether a dedicated RectangleCollection is justified.