[Bug]: Global legend weird behaviors

Bug summary

The color of the global legend for all subplots does not match the label colors.

Code for reproduction

esbs = np.array(list(range(0,41)))
lines = [e for e in asrs]

colors = ['dodgerblue', 'tomato', 'yellowgreen', 'orange', 'dodgerblue', 'tomato', 'yellowgreen', 'orange']
labels = ['A', 'C', 'M','F','A+', 'C+', 'M+','F+',]
linestyles = ['-', '-','-', '-','--','--','--', '--']

fig, axs = plt.subplots(2, figsize=(6, 6), sharex=False)

handles = []
for i in range(8):
    print(i, colors[i], labels[i])
    if i == 3 or i == 7:
        h = axs[1].plot(esbs, lines[i], color=colors[i], linewidth=1.2, linestyle=linestyles[i], label=labels[i])
    else:
        h = axs[0].plot(esbs, lines[i], color=colors[i], linewidth=1.2, linestyle=linestyles[i], label=labels[i])
    handles.append(h)

fig.legend(handles, labels=labels, ncol=2, bbox_to_anchor=(0.15, 1.06), loc='upper left')

plt.show()

Actual outcome

Expected outcome

The colors for legend F and legend F+ should be orange. But they are different.

Additional information

No response

Operating system

No response

Matplotlib Version

3.7.1

Matplotlib Backend

No response

Python version

No response

Jupyter version

No response

Installation

None

This looks like it is due to the signature on legend being too forgiving.

https://matplotlib.org/stable/api/figure_api.html#matplotlib.figure.Figure.legend

It looks like we try to infer what the positional argument means (it can be a list of strings to go with the auto-discovered artists if it is the only positional argument or a list of handles if there are two positional arguments). If you do not pass the labels as a keyword argument it will raise.

What is going on here is we are going through a branch of

matplotlib/lib/matplotlib/legend.py

Lines 1336 to 1374 in e6af5a3

    
           if (handles is not None or labels is not None) and args: 
        
               _api.warn_external("You have mixed positional and keyword arguments, " 
        
                                  "some input may be discarded.") 
        
           # if got both handles and labels as kwargs, make same length 
        
           if handles and labels: 
        
               handles, labels = zip(*zip(handles, labels)) 
        
           elif handles is not None and labels is None: 
        
               labels = [handle.get_label() for handle in handles] 
        
           elif labels is not None and handles is None: 
        
               # Get as many handles as there are labels. 
        
               handles = [handle for handle, label 
        
                          in zip(_get_legend_handles(axs, handlers), labels)] 
        
           elif len(args) == 0:  # 0 args: automatically detect labels and handles. 
        
               handles, labels = _get_legend_handles_labels(axs, handlers) 
        
               if not handles: 
        
                   log.warning( 
        
                       "No artists with labels found to put in legend.  Note that " 
        
                       "artists whose label start with an underscore are ignored " 
        
                       "when legend() is called with no argument.") 
        
           elif len(args) == 1:  # 1 arg: user defined labels, automatic handle detection. 
        
               labels, = args 
        
               if any(isinstance(l, Artist) for l in labels): 
        
                   raise TypeError("A single argument passed to legend() must be a " 
        
                                   "list of labels, but found an Artist in there.") 
        
               # Get as many handles as there are labels. 
        
               handles = [handle for handle, label 
        
                          in zip(_get_legend_handles(axs, handlers), labels)] 
        
           elif len(args) == 2:  # 2 args: user defined handles and labels. 
        
               handles, labels = args[:2] 
        
           else: 
        
               raise _api.nargs_error('legend', '0-2', len(args))

that never looks at *args so the list of handles is being ignored (!!) so are zipping the labels passed in with the list of discovered handles which will be by axes hence the two lines on the second axes will be together at the end.

You should be getting a warning that looks like

<ipython-input-12-f89da0bd97bd>:19: UserWarning: You have mixed positional and keyword arguments, some input may be discarded.
  fig.legend(handles, labels=labels, ncol=2, bbox_to_anchor=(0.15, 1.06), loc='upper left')

I think we should escalate that warning to an exception because we have been warning for at least 6 years (but without saying it would break in the future), it is really bad to drop user input on the floor, and this behavior is deeply miss leading.

In the mean time

fig.legend(handles, labels, ncol=2, bbox_to_anchor=(0.15, 1.06), loc='upper left')
fig.legend(handles=handles, labels=labels, ncol=2, bbox_to_anchor=(0.15, 1.06), loc='upper left')

both work correctly but he best invocation is probably

fig.legend(handles=handles, ncol=2, bbox_to_anchor=(0.15, 1.06), loc='upper lef

which lets the legend code extract the label from the Artist (which you set when you create the Artist so it can never gets out of sync).

Thank you for your detailed analysis and explanation. ---- Replied Message ----FromThomas A ***@***.***>Date09/20/2023 21:54 ***@***.***> CcT.X. ***@***.***>***@***.***>SubjectRe: [matplotlib/matplotlib] [Bug]: Global legend weird behaviors (Issue #26841) This looks like it is due to the signature on legend being too forgiving. https://matplotlib.org/stable/api/figure_api.html#matplotlib.figure.Figure.legend It looks like we try to infer what the positional argument means (it can be a list of strings to go with the auto-discovered artists if it is the only positional argument or a list of handles if there are two positional arguments). If you do not pass the labels as a keyword argument it will raise. What is going on here is we are going through a branch of https://github.com/matplotlib/matplotlib/blob/e6af5a339c7cea73fa0aaa587e258ecb742d4bef/lib/matplotlib/legend.py#L1336-L1374 that never looks at *args so the list of handles is being ignored (!!) so are zipping the labels passed in with the list of discovered handles which will be by axes hence the two lines on the second axes will be together at the end. You should be getting a warning that looks like <ipython-input-12-f89da0bd97bd>:19: UserWarning: You have mixed positional and keyword arguments, some input may be discarded. fig.legend(handles, labels=labels, ncol=2, bbox_to_anchor=(0.15, 1.06), loc='upper left') I think we should escalate that warning to an exception because we have been warning for at least 6 years (but without saying it would break in the future), it is really bad to drop user input on the floor, and this behavior is deeply miss leading. In the mean time fig.legend(handles, labels, ncol=2, bbox_to_anchor=(0.15, 1.06), loc='upper left') fig.legend(handles=handles, labels=labels, ncol=2, bbox_to_anchor=(0.15, 1.06), loc='upper left') both work correctly but he best invocation is probably fig.legend(handles=handles, ncol=2, bbox_to_anchor=(0.15, 1.06), loc='upper lef which lets the legend code extract the label from the Artist (which you set when you create the Artist so it can never gets out of sync). —Reply to this email directly, view it on GitHub, or unsubscribe.You are receiving this because you authored the thread.Message ID: ***@***.***> [ { ***@***.***": "http://schema.org", ***@***.***": "EmailMessage", "potentialAction": { ***@***.***": "ViewAction", "target": "#26841 (comment)", "url": "#26841 (comment)", "name": "View Issue" }, "description": "View this Issue on GitHub", "publisher": { ***@***.***": "Organization", "name": "GitHub", "url": "https://github.com" } } ]

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[Bug]: Global legend weird behaviors #26841

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	if (handles is not None or labels is not None) and args:
	_api.warn_external("You have mixed positional and keyword arguments, "
	"some input may be discarded.")

	# if got both handles and labels as kwargs, make same length
	if handles and labels:
	handles, labels = zip(*zip(handles, labels))

	elif handles is not None and labels is None:
	labels = [handle.get_label() for handle in handles]

	elif labels is not None and handles is None:
	# Get as many handles as there are labels.
	handles = [handle for handle, label
	in zip(_get_legend_handles(axs, handlers), labels)]

	elif len(args) == 0: # 0 args: automatically detect labels and handles.
	handles, labels = _get_legend_handles_labels(axs, handlers)
	if not handles:
	log.warning(
	"No artists with labels found to put in legend. Note that "
	"artists whose label start with an underscore are ignored "
	"when legend() is called with no argument.")

	elif len(args) == 1: # 1 arg: user defined labels, automatic handle detection.
	labels, = args
	if any(isinstance(l, Artist) for l in labels):
	raise TypeError("A single argument passed to legend() must be a "
	"list of labels, but found an Artist in there.")

	# Get as many handles as there are labels.
	handles = [handle for handle, label
	in zip(_get_legend_handles(axs, handlers), labels)]

	elif len(args) == 2: # 2 args: user defined handles and labels.
	handles, labels = args[:2]

	else:
	raise _api.nargs_error('legend', '0-2', len(args))

[Bug]: Global legend weird behaviors #26841

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Bug summary

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Actual outcome

Expected outcome

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tacaswell commented on Sep 20, 2023

imxtx commented on Sep 20, 2023

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