How do I make a single legend for many subplots?

Question

I am plotting the same type of information, but for different countries, with multiple subplots with Matplotlib. That is, I have nine plots on a 3x3 grid, all with the same for lines (of course, different values per line).

However, I have not figured out how to put a single legend (since all nine subplots have the same lines) on the figure just once.

How do I do that?

Answer 1

There is also a nice function get_legend_handles_labels() you can call on the last axis (if you iterate over them) that would collect everything you need from label= arguments:

handles, labels = ax.get_legend_handles_labels()
fig.legend(handles, labels, loc='upper center')

If the pyplot interface is being used instead of the Axes interface, use:

handles, labels = plt.gca().get_legend_handles_labels()

To remove legends from subplots, see Remove the legend on a matplotlib figure.

To merge twinx legends, see Secondary axis with twinx(): how to add to legend.

Answer 2

TL;DR

lines_labels = [ax.get_legend_handles_labels() for ax in fig.axes]
lines, labels = [sum(lol, []) for lol in zip(*lines_labels)]
fig.legend(lines, labels)

---

I have noticed that none of the other answers displays an image with a single legend referencing many curves in different subplots, so I have to show you one... to make you curious...

Now, if I've teased you enough, here it is the code

from numpy import linspace
import matplotlib.pyplot as plt

# each Axes has a brand new prop_cycle, so to have differently
# colored curves in different Axes, we need our own prop_cycle
# Note: we CALL the axes.prop_cycle to get an itertoools.cycle
color_cycle = plt.rcParams['axes.prop_cycle']()

# I need some curves to plot
x = linspace(0, 1, 51)
functs = [x*(1-x), x**2*(1-x),
          0.25-x*(1-x), 0.25-x**2*(1-x)] 
labels = ['$x-x²$', '$x²-x³$',
          '$\\frac{1}{4} - (x-x²)$', '$\\frac{1}{4} - (x²-x³)$']

# the plot, 
fig, (a1,a2) = plt.subplots(2)
for ax, f, l, cc in zip((a1,a1,a2,a2), functs, labels, color_cycle): 
    ax.plot(x, f, label=l, **cc)
    ax.set_aspect(2) # superfluos, but nice

# So far, nothing special except the managed prop_cycle. Now the trick:
lines_labels = [ax.get_legend_handles_labels() for ax in fig.axes]
lines, labels = [sum(lol, []) for lol in zip(*lines_labels)]

# Finally, the legend (that maybe you'll customize differently)
fig.legend(lines, labels, loc='upper center', ncol=4)
plt.show()

• If you want to stick with the official Matplotlib API, this is perfect, otherwise see note no.1 below (there is a private method...)

• The two lines

  lines_labels = [ax.get_legend_handles_labels() for ax in fig.axes]
  lines, labels = [sum(lol, []) for lol in zip(*lines_labels)]
  

  deserve an explanation, see note 2 below.

• I tried the method proposed by the most up-voted and accepted answer,

   # fig.legend(lines, labels, loc='upper center', ncol=4)
   fig.legend(*a2.get_legend_handles_labels(),
              loc='upper center', ncol=4)
  

  and this is what I've got

---

Note 1
If you don't mind using a private method of the matplotlib.legend module ... it's really much much much easier

from matplotlib.legend import _get_legend_handles_labels
...

fig.legend(*_get_legend_handles_and_labels(fig.axes), ...)

---

Note 2

I have encapsulated the two tricky lines in a function, just four lines of code, but heavily commented

def fig_legend(fig, **kwdargs):

    # Generate a sequence of tuples, each contains
    #  - a list of handles (lohand) and
    #  - a list of labels (lolbl)
    tuples_lohand_lolbl = (ax.get_legend_handles_labels() for ax in fig.axes)
    # E.g., a figure with two axes, ax0 with two curves, ax1 with one curve
    # yields:   ([ax0h0, ax0h1], [ax0l0, ax0l1]) and ([ax1h0], [ax1l0])

    # The legend needs a list of handles and a list of labels,
    # so our first step is to transpose our data,
    # generating two tuples of lists of homogeneous stuff(tolohs), i.e.,
    # we yield ([ax0h0, ax0h1], [ax1h0]) and ([ax0l0, ax0l1], [ax1l0])
    tolohs = zip(*tuples_lohand_lolbl)

    # Finally, we need to concatenate the individual lists in the two
    # lists of lists: [ax0h0, ax0h1, ax1h0] and [ax0l0, ax0l1, ax1l0]
    # a possible solution is to sum the sublists - we use unpacking
    handles, labels = (sum(list_of_lists, []) for list_of_lists in tolohs)

    # Call fig.legend with the keyword arguments, return the legend object

    return fig.legend(handles, labels, **kwdargs)

I recognize that sum(list_of_lists, []) is a really inefficient method to flatten a list of lists, but ① I love its compactness, ② usually is a few curves in a few subplots and ③ Matplotlib and efficiency? ;-)

Answer 3

For the automatic positioning of a single legend in a figure with many axes, like those obtained with subplots(), the following solution works really well:

plt.legend(lines, labels, loc = 'lower center', bbox_to_anchor = (0, -0.1, 1, 1),
           bbox_transform = plt.gcf().transFigure)

With bbox_to_anchor and bbox_transform=plt.gcf().transFigure, you are defining a new bounding box of the size of your figureto be a reference for loc. Using (0, -0.1, 1, 1) moves this bounding box slightly downwards to prevent the legend to be placed over other artists.

OBS: Use this solution after you use fig.set_size_inches() and before you use fig.tight_layout()

Answer 4

All of the previous answers are way over my head, at this state of my coding journey, so I just added another Matplotlib aspect called patches:

import matplotlib.patches as mpatches

first_leg = mpatches.Patch(color='red', label='1st plot')
second_leg = mpatches.Patch(color='blue', label='2nd plot')
thrid_leg = mpatches.Patch(color='green', label='3rd plot')
plt.legend(handles=[first_leg ,second_leg ,thrid_leg ])

The patches aspect put all the data i needed on my final plot (it was a line plot that combined three different line plots all in the same cell in Jupyter Notebook).

Result

(I changed the names form what I named my own legend.)

Answer 5

To build on top of gboffi's and Ben Usman's answer:

In a situation where one has different lines in different subplots with the same color and label, one can do something along the lines of:

labels_handles = {
  label: handle for ax in fig.axes for handle, label in zip(*ax.get_legend_handles_labels())
}

fig.legend(
  labels_handles.values(),
  labels_handles.keys(),
  loc = "upper center",
  bbox_to_anchor = (0.5, 0),
  bbox_transform = plt.gcf().transFigure,
)

Answer 6

Using Matplotlib 2.2.2, this can be achieved using the gridspec feature.

In the example below, the aim is to have four subplots arranged in a 2x2 fashion with the legend shown at the bottom. A 'faux' axis is created at the bottom to place the legend in a fixed spot. The 'faux' axis is then turned off so only the legend shows. Result:

import matplotlib.pyplot as plt
import matplotlib.gridspec as gridspec

# Gridspec demo
fig = plt.figure()
fig.set_size_inches(8, 9)
fig.set_dpi(100)

rows   = 17 # The larger the number here, the smaller the spacing around the legend
start1 = 0
end1   = int((rows-1)/2)
start2 = end1
end2   = int(rows-1)

gspec = gridspec.GridSpec(ncols=4, nrows=rows)

axes = []
axes.append(fig.add_subplot(gspec[start1:end1, 0:2]))
axes.append(fig.add_subplot(gspec[start2:end2, 0:2]))
axes.append(fig.add_subplot(gspec[start1:end1, 2:4]))
axes.append(fig.add_subplot(gspec[start2:end2, 2:4]))
axes.append(fig.add_subplot(gspec[end2, 0:4]))

line, = axes[0].plot([0, 1], [0, 1], 'b')         # Add some data
axes[-1].legend((line,), ('Test',), loc='center') # Create legend on bottommost axis
axes[-1].set_axis_off()                           # Don't show the bottom-most axis

fig.tight_layout()
plt.show()

Answer 7

If you are using subplots with bar charts, with a different colour for each bar, it may be faster to create the artefacts yourself using mpatches.

Say you have four bars with different colours as r, m, c, and k, you can set the legend as follows:

import matplotlib.patches as mpatches
import matplotlib.pyplot as plt
labels = ['Red Bar', 'Magenta Bar', 'Cyan Bar', 'Black Bar']


#####################################
# Insert code for the subplots here #
#####################################


# Now, create an artist for each color
red_patch = mpatches.Patch(facecolor='r', edgecolor='#000000') # This will create a red bar with black borders, you can leave out edgecolor if you do not want the borders
black_patch = mpatches.Patch(facecolor='k', edgecolor='#000000')
magenta_patch = mpatches.Patch(facecolor='m', edgecolor='#000000')
cyan_patch = mpatches.Patch(facecolor='c', edgecolor='#000000')
fig.legend(handles = [red_patch, magenta_patch, cyan_patch, black_patch], labels=labels,
       loc="center right",
       borderaxespad=0.1)
plt.subplots_adjust(right=0.85) # Adjust the subplot to the right for the legend

Answer 8

figlegend may be what you're looking for: matplotlib.pyplot.figlegend

An example is at Figure legend demo.

Another example:

plt.figlegend(lines, labels, loc = 'lower center', ncol=5, labelspacing=0.)

Or:

fig.legend(lines, labels, loc = (0.5, 0), ncol=5)

Answer 9

This answer is a complement to user707650's answer on the legend position.

My first try on user707650's solution failed due to overlaps of the legend and the subplot's title.

In fact, the overlaps are caused by fig.tight_layout(), which changes the subplots' layout without considering the figure legend. However, fig.tight_layout() is necessary.

In order to avoid the overlaps, we can tell fig.tight_layout() to leave spaces for the figure's legend by fig.tight_layout(rect=(0,0,1,0.9)).

Description of tight_layout() parameters.

Answer 10

You just have to ask for the legend once, outside of your loop.

For example, in this case I have 4 subplots, with the same lines, and a single legend.

from matplotlib.pyplot import *

ficheiros = ['120318.nc', '120319.nc', '120320.nc', '120321.nc']

fig = figure()
fig.suptitle('concentration profile analysis')

for a in range(len(ficheiros)):
    # dados is here defined
    level = dados.variables['level'][:]

    ax = fig.add_subplot(2,2,a+1)
    xticks(range(8), ['0h','3h','6h','9h','12h','15h','18h','21h']) 
    ax.set_xlabel('time (hours)')
    ax.set_ylabel('CONC ($\mu g. m^{-3}$)')

    for index in range(len(level)):
        conc = dados.variables['CONC'][4:12,index] * 1e9
        ax.plot(conc,label=str(level[index])+'m')

    dados.close()

ax.legend(bbox_to_anchor=(1.05, 0), loc='lower left', borderaxespad=0.)
         # it will place the legend on the outer right-hand side of the last axes

show()