Secondary axis with twinx how to add to legend

Question

I have a plot with two y-axes  using twinx    I also give labels to the lines  and want to show them with legend    but I only succeed to get the labels of one axis in the legend   import numpy as np import matplotlib pyplot as plt from matplotlib import rc rc  mathtext   default  regular    fig   plt figure   ax   fig add subplot 111  ax plot time  Swdown   -   label    Swdown   ax plot time  Rn   -   label    Rn   ax2   ax twinx   ax2 plot time  temp   -r   label    temp   ax legend loc 0  ax grid   ax set xlabel  Time  h    ax set ylabel r Radiation   MJ  m  -2   d  -1      ax2 set ylabel r Temperature     circ C    ax2 set ylim 0  35  ax set ylim -20 100  plt show     So I only get the labels of the first axis in the legend  and not the label  temp  of the second axis  How could I add this third label to the legend

User · Answer

From matplotlib version 2 1 onwards  you may use a figure legend  Instead of ax legend    which produces a legend with the handles from the axes ax  one can create a figure legend  fig legend loc  upper right    which will gather all handles from all subplots in the figure  Since it is a figure legend  it will be placed at the corner of the figure  and the loc argument is relative to the figure   import numpy as np import matplotlib pyplot as plt  x   np linspace 0 10  y   np linspace 0 10  z   np sin x 3   2 98  fig   plt figure   ax   fig add subplot 111  ax plot x y   -   label    Quantity 1    ax2   ax twinx   ax2 plot x z   -r   label    Quantity 2   fig legend loc  upper right    ax set xlabel  x  units    ax set ylabel r Quantity 1   ax2 set ylabel r Quantity 2    plt show       In order to place the legend back into the axes  one would supply a bbox to anchor and a bbox transform  The latter would be the axes transform of the axes the legend should reside in  The former may be the coordinates of the edge defined by loc given in axes coordinates   fig legend loc  upper right   bbox to anchor  1 1   bbox transform ax transAxes

User · Answer

I m not sure if this functionality is new  but you can also use the get legend handles labels   method rather than keeping track of lines and labels yourself   import numpy as np import matplotlib pyplot as plt from matplotlib import rc rc  mathtext   default  regular    pi   np pi    fake data time   np linspace  0  25  50  temp   50   np sqrt  2   pi   3  2              np exp  -  time - 13   2    3  2    2    15 Swdown   400   np sqrt  2   pi   3  2    np exp  -  time - 13   2    3  2    2  Rn   Swdown - 10  fig   plt figure   ax   fig add subplot 111   ax plot time  Swdown   -   label    Swdown   ax plot time  Rn   -   label    Rn   ax2   ax twinx   ax2 plot time  temp   -r   label    temp      ask matplotlib for the plotted objects and their labels lines  labels   ax get legend handles labels   lines2  labels2   ax2 get legend handles labels   ax2 legend lines   lines2  labels   labels2  loc 0   ax grid   ax set xlabel  Time  h    ax set ylabel r Radiation   MJ  m  -2   d  -1      ax2 set ylabel r Temperature     circ C    ax2 set ylim 0  35  ax set ylim -20 100  plt show

User · Answer

You can easily add a second legend by adding the line   ax2 legend loc 0    You ll get this     But if you want all labels on one legend then you should do something like this   import numpy as np import matplotlib pyplot as plt from matplotlib import rc rc  mathtext   default  regular    time   np arange 10  temp   np random random 10  30 Swdown   np random random 10  100-10 Rn   np random random 10  100-10  fig   plt figure   ax   fig add subplot 111   lns1   ax plot time  Swdown   -   label    Swdown   lns2   ax plot time  Rn   -   label    Rn   ax2   ax twinx   lns3   ax2 plot time  temp   -r   label    temp      added these three lines lns   lns1 lns2 lns3 labs    l get label   for l in lns  ax legend lns  labs  loc 0   ax grid   ax set xlabel  Time  h    ax set ylabel r Radiation   MJ  m  -2   d  -1      ax2 set ylabel r Temperature     circ C    ax2 set ylim 0  35  ax set ylim -20 100  plt show     Which will give you this

User · Answer

I found an following official matplotlib example that uses host subplot to display multiple y-axes and all the different labels in one legend  No workaround necessary  Best solution I found so far   http   matplotlib org examples axes grid demo parasite axes2 html   from mpl toolkits axes grid1 import host subplot import mpl toolkits axisartist as AA import matplotlib pyplot as plt  host   host subplot 111  axes class AA Axes  plt subplots adjust right 0 75   par1   host twinx   par2   host twinx    offset   60 new fixed axis   par2 get grid helper   new fixed axis par2 axis  right     new fixed axis loc  right                                       axes par2                                      offset  offset  0    par2 axis  right   toggle all True   host set xlim 0  2  host set ylim 0  2   host set xlabel  Distance   host set ylabel  Density   par1 set ylabel  Temperature   par2 set ylabel  Velocity    p1    host plot  0  1  2    0  1  2   label  Density   p2    par1 plot  0  1  2    0  3  2   label  Temperature   p3    par2 plot  0  1  2    50  30  15   label  Velocity    par1 set ylim 0  4  par2 set ylim 1  65   host legend    plt draw   plt show

User · Answer

You can easily get what you want by adding the line in ax   ax plot          -r   label    temp     or   ax plot np nan   -r   label    temp     This would plot nothing but add a label to legend of ax   I think this is a much easier way  It s not necessary to track lines automatically when you have only a few lines in the second axes  as fixing by hand like above would be quite easy  Anyway  it depends on what you need   The whole code is as below   import numpy as np import matplotlib pyplot as plt from matplotlib import rc rc  mathtext   default  regular    time   np arange 22   temp   20 np random rand 22  Swdown   10 np random randn 22  40 Rn   40 np random rand 22   fig   plt figure   ax   fig add subplot 111  ax2   ax twinx     ---------- look at below -----------  ax plot time  Swdown   -   label    Swdown   ax plot time  Rn   -   label    Rn    ax2 plot time  temp   -r      The true line in ax2 ax plot np nan   -r   label    temp      Make an agent in ax  ax legend loc 0    ---------------done-----------------  ax grid   ax set xlabel  Time  h    ax set ylabel r Radiation   MJ  m  -2   d  -1      ax2 set ylabel r Temperature     circ C    ax2 set ylim 0  35  ax set ylim -20 100  plt show     The plot is as below       Update  add a better version   ax plot np nan   -r   label    temp     This will do nothing while plot 0  0  may change the axis range     One extra example for scatter  ax scatter         s 100  label    temp      Make an agent in ax ax2 scatter time  temp  s 10     The true scatter in ax2  ax legend loc 1  framealpha 1

User · Answer

A quick hack that may suit your needs    Take off the frame of the box and manually position the two legends next to each other  Something like this    ax1 legend loc     75  1   frameon   False  ax2 legend  loc     75   05   frameon   False    Where the loc tuple is left-to-right and bottom-to-top percentages that represent the location in the chart

[python] Secondary axis with twinx(): how to add to legend?

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