how do I make a single legend for many subplots with matplotlib

Question

I am plotting the same type of information  but for different countries  with multiple subplots with matplotlib  That is  I have 9 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 9 subplots have the same lines  on the figure just once    How do I do that

User · Answer

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()

User · Answer

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 bouding 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

User · Answer

figlegend may be what you re looking for  http   matplotlib org api pyplot api html matplotlib pyplot figlegend  Example here  http   matplotlib org examples pylab examples figlegend demo html  Another example   plt figlegend  lines  labels  loc    lower center   ncol 5  labelspacing 0      or   fig legend  lines  labels  loc    0 5  0   ncol 5

User · Answer

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

User · Answer

I have noticed that no answer display an image  with a single legend referencing many curves in different subplots  so I have to show you one    to make you curious     Now  you want to look at the code  don t you  from numpy import linspace import matplotlib pyplot as plt    Calling the axes prop cycle returns an itertoools cycle  color cycle   plt rcParams  axes prop cycle        I need some curves to plot  x   linspace 0  1  51  f1   x  1-x      lab1    x - x x  f2   0 25-f1     lab2    1 4 - x   x x   f3   x x  1-x    lab3    x x - x x x  f4   0 25-f3     lab4    1 4 - x x   x x x     let s plot our curves  note the use of color cycle  otherwise the curves colors in   the two subplots will be repeated and a single legend becomes difficult to read  fig   a13  a24    plt subplots 2   a13 plot x  f1  label lab1    next color cycle   a13 plot x  f3  label lab3    next color cycle   a24 plot x  f2  label lab2    next color cycle   a24 plot x  f4  label lab4    next color cycle      so far so good  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 we invoke the legend  that you probably would like to customize      fig legend lines  labels  plt show    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     to this aim I have encapsulated the tricky part in a function  just 4 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              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    PS I recognize that sum list of lists      is a really inefficient method to flatten a list of lists but   I love its compactness    usually is a few curves in a few subplots and   Matplotlib and efficiency   -   Important Update If you want to stick with the official Matplotlib API my answer above is perfect  really  On the other hand  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         A complete explanation can be found in the source code of Axes get legend handles labels in     matplotlib axes  axes py

User · Answer

This answer is a complement to  Evert s on the legend position   My first try on  Evert 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

User · Answer

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

User · Answer

While rather late to the game  I ll give another solution here as this is still one of the first links to show up on google  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  https   i stack imgur com 5LUWM png   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 bottommost axis  fig tight layout   plt show

User · Answer

if you are using subplots with bar charts  with 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 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

[python] how do I make a single legend for many subplots with matplotlib?

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