hist reverses legend order if histtype is not bar

[efiring]

Matplotlib v2.0.x and master on Python 3.5:

x1 = np.random.randn(100)
x2 = np.random.rand(100)
plt.hist([x1, x2], label=['x1', 'x2'], histtype='step')
plt.legend()

Actual outcome

Expected outcome

With histtype='bar' the order of the variables is preserved, as I would expect, instead of reversed, as with step.

[anntzer]

The culprit is the comment at the top of

            # add patches in reverse order so that when stacking,
            # items lower in the stack are plottted on top of
            # items higher in the stack
            for x, y, c in reversed(list(zip(xvals, yvals, color))):
                patches.append(self.fill(
                    x[:split], y[:split],
                    closed=True if fill else None,
                    facecolor=c,
                    edgecolor=None if fill else c,
                    fill=fill if fill else None))
            for patch_list in patches:
                for patch in patch_list:
                    if orientation == 'vertical':
                        patch.sticky_edges.y.append(minimum)
                    elif orientation == 'horizontal':
                        patch.sticky_edges.x.append(minimum)

            # we return patches, so put it back in the expected order
            patches.reverse()

[anntzer]

I think any reasonable fix to this will lead to inversing the order in which the bars are being drawn (which is fine with me -- you'd expect the later datasets to be plotted on top of the earlier ones per standard matplotlib behavior -- but would definitely cause a backcompatibility break).

[davidjamesweir]

This bug is still present in matplotlib 3.7.1 and python 3.11.2. For the reason given by @anntzer above, it becomes even more confusing if one calls plt.legend() separately:

x1 = np.random.randn(100)
x2 = np.random.rand(100)
plt.hist([x1, x2], histtype='step')
plt.legend(['x1', 'x2'])

yields reversed colours and labels:

It's easy to see how this bug could fly under the radar: the swapping of colours and labels in code for a work project this morning confused me for quite some time.

[tacaswell]

I believe this is partially addressed by #24759, adjusting @efiring 's example to use plt.legend(reverse=True). Not ideal, but it is something.

The example from @davidjamesweir is a less than great feature of the API where you can pass ax.legend just the labels you want and then we go and look for labelable things and assign the labels in that order. If the plotting calls only produce one Artist this is clear, but if our calls (or a third-party library calls) produce more than one Artist you are now exposed to a slightly fuzzy aspect of our API (is the order we add Artists to the internal lists stable or not?!).

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[timhoffm]

The reversal dates back to 2e1a96d1, but I see no obvious reason why that has been done.

One could retain the draw order while reversing the lists by adding minimal zorder (e.g. 1e-12). But that's a bit hacky, and also the draw order being different between types bar and stepfilled is quite surprising. I would consider this buggy behavior even though it seems to have been introduced intentionally. In that sense I'd be fine with just reversing the order and calling it a bug fix.

[Rainald62]

Time to fix this bug. I guess that most existing code is no longer in use. Suggestion: Let the parameter reverse have None as default = reversing for stacked histograms only, so that most existing code could be fixed by just removing the reverse parameter. Emit a warning for a year or so.

[timhoffm]

That migration pattern does not really help much: We've introduced legend(reverse=...) as a general-purpose parameter. It can be used to fix the order of hist, but also generally. To make a smoother hist-migration, we'd have to try and detect whether legend(reverse=True) was used to fix the hist order, i.e. some internal book-keeping that the legend entries all come from a hist call. While it's technically possible IMHO it's not worth the effort.

The slightly better approach would be to introduce a reverse_legend_order parameter (name t.b.d.) on hist, so that we contain the topic of artist ordering in hist().