Add a common Legend for combined ggplots

Question

I have two ggplots which I align horizontally with grid arrange  I have looked through a lot of forum posts  but everything I try seem to be commands that are now updated and named something else    My data looks like this     Data plot 1                                            axis1     axis2    group1 -0 212201  0 358867 group2 -0 279756 -0 126194 group3  0 186860 -0 203273 group4  0 417117 -0 002592 group1 -0 212201  0 358867 group2 -0 279756 -0 126194 group3  0 186860 -0 203273 group4  0 186860 -0 203273    Data plot 2            axis1     axis2 group1  0 211826 -0 306214 group2 -0 072626  0 104988 group3 -0 072626  0 104988 group4 -0 072626  0 104988 group1  0 211826 -0 306214 group2 -0 072626  0 104988 group3 -0 072626  0 104988 group4 -0 072626  0 104988   And I run this  library ggplot2  library gridExtra    groups c  group1   group2   group3   group4   group1   group2   group3   group4    x1 data1  1  y1 data1  2   x2 data2  1  y2 data2  2   p1 ggplot data1  aes x x1  y y1 colour groups     geom point position position jitter w 0 04 h 0 02  size 1 8   p2 ggplot data2  aes x x2  y y2 colour groups     geom point position position jitter w 0 04 h 0 02  size 1 8    Combine plots p3 grid arrange  p1   theme legend position  none    p2  theme legend position  none    nrow 1  widths   unit c 10  10    cm    heights   unit rep 8  1    cm       How would I extract the legend from any of these plots and add it to the bottom centre of the combined plot

User · Accepted Answer

Update 2015-Feb  See Steven s answer below    df1  lt - read table text  group   x     y    group1 -0 212201  0 358867 group2 -0 279756 -0 126194 group3  0 186860 -0 203273 group4  0 417117 -0 002592 group1 -0 212201  0 358867 group2 -0 279756 -0 126194 group3  0 186860 -0 203273 group4  0 186860 -0 203273  header TRUE   df2  lt - read table text  group   x     y    group1  0 211826 -0 306214 group2 -0 072626  0 104988 group3 -0 072626  0 104988 group4 -0 072626  0 104988 group1  0 211826 -0 306214 group2 -0 072626  0 104988 group3 -0 072626  0 104988 group4 -0 072626  0 104988  header TRUE    library ggplot2  library gridExtra   p1  lt - ggplot df1  aes x x  y y colour group     geom point position position jitter w 0 04 h 0 02  size 1 8    theme legend position  bottom    p2  lt - ggplot df2  aes x x  y y colour group     geom point position position jitter w 0 04 h 0 02  size 1 8    extract legend  https   github com hadley ggplot2 wiki Share-a-legend-between-two-ggplot2-graphs g legend lt -function a gplot     tmp  lt - ggplot gtable ggplot build a gplot     leg  lt - which sapply tmp grobs  function x  x name      guide-box     legend  lt - tmp grobs  leg     return legend    mylegend lt -g legend p1   p3  lt - grid arrange arrangeGrob p1   theme legend position  none                             p2   theme legend position  none                             nrow 1                mylegend  nrow 2 heights c 10  1     Here is the resulting plot

User · Answer

A new  attractive solution is to use patchwork  The syntax is very simple   library ggplot2  library patchwork   p1  lt - ggplot df1  aes x   x  y   y  colour   group        geom point position   position jitter w   0 04  h   0 02   size   1 8  p2  lt - ggplot df2  aes x   x  y   y  colour   group        geom point position   position jitter w   0 04  h   0 02   size   1 8   combined  lt - p1   p2  amp  theme legend position    bottom   combined   plot layout guides    collect       Created on 2019-12-13 by the reprex package  v0 2 1

User · Answer

Guiseppe    I have no idea of Grobs etc whatsoever  but I hacked together a solution for two plots  should be possible to extend to arbitrary number but its not in a sexy function   plots  lt - list p1  p2  g  lt - ggplotGrob plots  1     theme legend position  bottom    grobs legend  lt - g  which sapply g  function x  x name      guide-box     lheight  lt - sum legend height  tmp  lt - arrangeGrob p1   theme legend position    none    p2   theme legend position    none    layout matrix   matrix c 1  2   nrow   1   grid arrange tmp  legend  ncol   1  heights   unit c unit 1   npc   - lheight  lheight

User · Answer

If you are plotting the same variables in both plots  the simplest way would be to combine the data frames into one  then use facet wrap   For your example   big df  lt - rbind df1 df2   big df  lt - data frame big df Df   rep c  df1   df2    times c nrow df1  nrow df2      ggplot big df aes x x  y y colour group      geom point position position jitter w 0 04 h 0 02  size 1 8     facet wrap  Df      Another example using the diamonds data set  This shows that you can even make it work if you have only one variable common between your plots   diamonds reshaped  lt - data frame price   diamonds price  independent variable   c diamonds carat diamonds cut diamonds color diamonds depth   Clarity   rep diamonds clarity times 4   Variable name   rep c  Carat   Cut   Color   Depth   each nrow diamonds     ggplot diamonds reshaped aes independent variable price colour Clarity      geom point size 2    facet wrap  Variable name scales  free x      xlab         Only tricky thing with the second example is that the factor variables get coerced to numeric when you combine everything into one data frame  So ideally  you will do this mainly when all your variables of interest are the same type

User · Answer

If the legend is the same for both plots  there is a simple solution using grid arrange assuming you want your legend to align with both plots either vertically or horizontally   Simply keep the legend for the bottom-most or right-most plot while omitting the legend for the other  Adding a legend to just one plot  however  alters the size of one plot relative to the other  To avoid this use the heights command to manually adjust and keep them the same size  You can even use grid arrange to make common axis titles  Note that this will require library grid  in addition to library gridExtra   For vertical plots  y title  lt - expression paste italic  quot E  coli quot     quot   CFU 100mL  quot     grid arrange arrangeGrob p1  theme legend position  quot none quot    ncol 1       arrangeGrob p2  theme legend position  quot bottom quot    ncol 1       heights c 1 1 2   left textGrob y title  rot 90  gp gpar fontsize 20     Here is the result for a similar graph for a project I was working on

User · Answer

You may also use ggarrange from ggpubr package and set  common legend   TRUE    library ggpubr   dsamp  lt - diamonds sample nrow diamonds   1000     p1  lt - qplot carat  price  data   dsamp  colour   clarity  p2  lt - qplot cut  price  data   dsamp  colour   clarity  p3  lt - qplot color  price  data   dsamp  colour   clarity  p4  lt - qplot depth  price  data   dsamp  colour   clarity    ggarrange p1  p2  p3  p4  ncol 2  nrow 2  common legend   TRUE  legend  bottom

User · Answer

I suggest using cowplot  From their R vignette     load cowplot library cowplot     down-sampled diamonds data set dsamp  lt - diamonds sample nrow diamonds   1000        Make three plots    We set left and right margins to 0 to remove unnecessary spacing in the   final plot arrangement  p1  lt - qplot carat  price  data dsamp  colour clarity       theme plot margin   unit c 6 0 6 0    pt    p2  lt - qplot depth  price  data dsamp  colour clarity       theme plot margin   unit c 6 0 6 0    pt      ylab     p3  lt - qplot color  price  data dsamp  colour clarity       theme plot margin   unit c 6 0 6 0    pt      ylab        arrange the three plots in a single row prow  lt - plot grid  p1   theme legend position  none               p2   theme legend position  none               p3   theme legend position  none               align    vh              labels   c  A    B    C               hjust   -1             nrow   1                 extract the legend from one of the plots    clearly the whole thing only makes sense if all plots   have the same legend  so we can arbitrarily pick one   legend b  lt - get legend p1   theme legend position  bottom       add the legend underneath the row we made earlier  Give it 10  of the height   of one plot  via rel heights   p  lt - plot grid  prow  legend b  ncol   1  rel heights   c 1   2   p

User · Answer

Giuseppe  you may want to consider this for a flexible specification of the plots arrangement  modified from here    library ggplot2  library gridExtra  library grid   grid arrange shared legend  lt - function      nrow   1  ncol   length list        position   c  bottom    right         plots  lt - list        position  lt - match arg position    g  lt - ggplotGrob plots  1     theme legend position   position   grobs   legend  lt - g  which sapply g  function x  x name      guide-box       lheight  lt - sum legend height    lwidth  lt - sum legend width    gl  lt - lapply plots  function x  x   theme legend position    none      gl  lt - c gl  nrow   nrow  ncol   ncol     combined  lt - switch position                        bottom    arrangeGrob do call arrangeGrob  gl                                               legend                                              ncol   1                                              heights   unit c unit 1   npc   - lheight  lheight                          right    arrangeGrob do call arrangeGrob  gl                                              legend                                             ncol   2                                             widths   unit c unit 1   npc   - lwidth  lwidth      grid newpage     grid draw combined       Extra arguments nrow and ncol control the layout of the arranged plots   dsamp  lt - diamonds sample nrow diamonds   1000     p1  lt - qplot carat  price  data   dsamp  colour   clarity  p2  lt - qplot cut  price  data   dsamp  colour   clarity  p3  lt - qplot color  price  data   dsamp  colour   clarity  p4  lt - qplot depth  price  data   dsamp  colour   clarity  grid arrange shared legend p1  p2  p3  p4  nrow   1  ncol   4  grid arrange shared legend p1  p2  p3  p4  nrow   2  ncol   2

User · Answer

Roland s answer needs updating  See  https   github com hadley ggplot2 wiki Share-a-legend-between-two-ggplot2-graphs  This method has been updated for ggplot2 v1 0 0   library ggplot2  library gridExtra  library grid    grid arrange shared legend  lt - function            plots  lt - list          g  lt - ggplotGrob plots  1     theme legend position  bottom    grobs     legend  lt - g  which sapply g  function x  x name      guide-box         lheight  lt - sum legend height      grid arrange          do call arrangeGrob  lapply plots  function x              x   theme legend position  none              legend          ncol   1          heights   unit c unit 1   npc   - lheight  lheight      dsamp  lt - diamonds sample nrow diamonds   1000     p1  lt - qplot carat  price  data dsamp  colour clarity  p2  lt - qplot cut  price  data dsamp  colour clarity  p3  lt - qplot color  price  data dsamp  colour clarity  p4  lt - qplot depth  price  data dsamp  colour clarity  grid arrange shared legend p1  p2  p3  p4    Note the lack of ggplot gtable and ggplot build  ggplotGrob is used instead  This example is a bit more convoluted than the above solution but it still solved it for me

[r] Add a common Legend for combined ggplots

Update 2015-Feb

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