Add legend to ggplot2 line plot

Question

I have a question about legends in ggplot2  I managed to plot three lines in the same graph and want to add a legend with the three colors used  This is the code used  library ggplot2      require RCurl   link lt -getURL  https   dl dropbox com s ds5zp9jonznpuwb dat txt   datos lt - read csv textConnection link  header TRUE sep      datos fecha  lt - as POSIXct datos  1   format   d  m  Y        temp   ggplot data datos aes x fecha  y TempMax colour  1                  geom line colour  red     opts title  TITULO                ylab  Temperatura  C      xlab                    scale y continuous limits   c -10 40                 geom line aes x fecha  y TempMedia colour  2   colour  green                 geom line aes x fecha  y TempMin colour  2   colour  blue                scale colour manual values c  red   green   blue     temp   and the output    I d like to add a legend with the three colours used and the name of the variable  TempMax TempMedia and TempMin   I have tried   scale colour manual   but can t find the exact way   Unfortunately original data were deleted from linked site and could not be recovered  But they came from meteo data files with this format   date   Tmax   Tmin   Tmed   Precip diaria   Wmax   Wmed  2000-07-31 00 00 00 -1 7 -1 7 -1 7 -99 9 20 4 20 4 2000-08-01 00 00 00 22 9 19 21 11 -99 9 6 3 2 83 2000-08-03 00 00 00 24 8 12 3 19 23 -99 9 6 8 3 87 2000-08-04 00 00 00 20 3 9 4 14 4 -99 9 8 3 5 29 2000-08-08 00 00 00 25 7 14 4 19 5 -99 9 7 9 3 22 2000-08-09 00 00 00 29 8 16 2 22 14 -99 9 8 5 3 27 2000-08-10 00 00 00 30 17 8 23 5 -99 9 7 7 3 61 2000-08-11 00 00 00 27 5 17 22 68 -99 9 8 8 3 85 2000-08-12 00 00 00 24 13 3 17 32 -99 9 8 4 3 49

User · Answer

I really like the solution proposed by  Brian Diggs  However  in my case  I create the line plots in a loop rather than giving them explicitly because I do not know apriori how many plots I will have  When I tried to adapt the  Brian s code I faced some problems with handling the colors correctly  Turned out I needed to modify the aesthetic functions  In case someone has the same problem  here is the code that worked for me    I used the same data frame as  Brian   data  lt - structure list month   structure c 1317452400  1317538800  1317625200  1317711600                                          1317798000  1317884400  1317970800  1318057200                                          1318143600  1318230000  1318316400  1318402800                                          1318489200  1318575600  1318662000  1318748400                                          1318834800  1318921200  1319007600  1319094000                                         class   c  POSIXct    POSIXt    tzone                           TempMax   c 26 58  27 78  27 9  27 44  30 9  30 44  27 57  25 71                                  25 98  26 84  33 58  30 7  31 3  27 18  26 58  26 18                                  25 19  24 19  27 65  23 92                       TempMed   c 22 88  22 87  22 41  21 63  22 43  22 29  21 89  20 52                                   19 71  20 73  23 51  23 13  22 95  21 95  21 91  20 72                                    20 45  19 42  19 97  19 61                       TempMin   c 19 34  19 14  18 34  17 49  16 75  16 75  16 88  16 82                                  14 82  16 01  16 88  17 55  16 75  17 22  19 01  16 95                                  17 55  15 21  14 22  16 42                    Names   c  month    TempMax    TempMed    TempMin                   row names   c NA  20L   class    data frame       In my case  I generate my cols and my names dynamically  but I don t want to make things unnecessarily complicated so I give them explicitly here  These three lines make the ordering of the legend and assigning colors easier   my cols  lt - heat colors 3  alpha 1  my names  lt - c  TempMin    TempMed    TempMax   names my cols   lt - my names   And here is the plot   p  lt -  ggplot data  aes x   month    for  i in 1 3     p  lt - p   geom line aes  y   as name names data i 1     colour    colnames data i 1     as character my names i       p   scale colour manual                              breaks   as character my names                           values   my cols  p

User · Answer

I tend to find that if I m specifying individual colours in multiple geom s  I m doing it wrong  Here s how I would plot your data     Subset the necessary columns dd sub   datos  c 20  2 3 5     Then rearrange your data frame library reshape2  dd   melt dd sub  id c  fecha      All that s left is a simple ggplot command   ggplot dd    geom line aes x fecha  y value  colour variable       scale colour manual values c  red   green   blue      Example plot

User · Answer

Since  Etienne asked how to do this without melting the data  which in general is the preferred method  but I recognize there may be some cases where that is not possible   I present the following alternative   Start with a subset of the original data   datos  lt - structure list fecha   structure c 1317452400  1317538800  1317625200   1317711600  1317798000  1317884400  1317970800  1318057200  1318143600   1318230000  1318316400  1318402800  1318489200  1318575600  1318662000   1318748400  1318834800  1318921200  1319007600  1319094000   class   c  POSIXct     POSIXt    tzone        TempMax   c 26 58  27 78  27 9  27 44   30 9  30 44  27 57  25 71  25 98  26 84  33 58  30 7  31 3  27 18   26 58  26 18  25 19  24 19  27 65  23 92   TempMedia   c 22 88   22 87  22 41  21 63  22 43  22 29  21 89  20 52  19 71  20 73   23 51  23 13  22 95  21 95  21 91  20 72  20 45  19 42  19 97   19 61   TempMin   c 19 34  19 14  18 34  17 49  16 75  16 75   16 88  16 82  14 82  16 01  16 88  17 55  16 75  17 22  19 01   16 95  17 55  15 21  14 22  16 42     Names   c  fecha    TempMax     TempMedia    TempMin    row names   c NA  20L   class    data frame     You can get the desired effect by  and this also cleans up the original plotting code    ggplot data   datos  aes x   fecha       geom line aes y   TempMax  colour    TempMax        geom line aes y   TempMedia  colour    TempMedia        geom line aes y   TempMin  colour    TempMin        scale colour manual                            breaks   c  TempMax    TempMedia    TempMin                          values   c  red    green    blue        xlab          scale y continuous  Temperatura  C    limits   c -10 40        labs title  TITULO     The idea is that each line is given a color by mapping the colour aesthetic to a constant string  Choosing the string which is what you want to appear in the legend is the easiest  The fact that in this case it is the same as the name of the y variable being plotted is not significant  it could be any set of strings  It is very important that this is inside the aes call   you are creating a mapping to this  variable    scale colour manual can now map these strings to the appropriate colors  The result is   In some cases  the mapping between the levels and colors needs to be made explicit by naming the values in the manual scale  thanks to  DaveRGP for pointing this out    ggplot data   datos  aes x   fecha       geom line aes y   TempMax  colour    TempMax        geom line aes y   TempMedia  colour    TempMedia        geom line aes y   TempMin  colour    TempMin        scale colour manual                            values   c  TempMedia   green    TempMax   red                                      TempMin   blue        xlab          scale y continuous  Temperatura  C    limits   c -10 40        labs title  TITULO      giving the same figure as before   With named values  the breaks can be used to set the order in the legend and any order can be used in the values   ggplot data   datos  aes x   fecha       geom line aes y   TempMax  colour    TempMax        geom line aes y   TempMedia  colour    TempMedia        geom line aes y   TempMin  colour    TempMin        scale colour manual                            breaks   c  TempMedia    TempMax    TempMin                          values   c  TempMedia   green    TempMax   red                                      TempMin   blue        xlab          scale y continuous  Temperatura  C    limits   c -10 40        labs title  TITULO

[r] Add legend to ggplot2 line plot

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