Adding a legend to a combined line and bargraph ggplot

问题

So I know many people have asked similar questions but the code others have used does not seem to be working for my graph hence why I'm wondering if I have done something wrong.

I have this code:

ggplot(dfMonth) 
+ geom_col(aes(x=Month, y=NumberMO), size=.7, colour="black", fill="white") 
+ geom_line(aes(x=Month, y=NumberME), size=1, colour="black", group=1)  
+ xlab("Month") 
+ ylab("No. of birds observed") 
+ theme_bw() 
+ geom_point(x=Month, y=NumberME) 
+ scale_colour_manual("" ,values =c("NumberME"="black"), labels=c("Expected No. of birds")) 
+ theme(legend.key=element_blank(),legend.title=element_blank(), legend.box="horizontal") 
+ theme(axis.title.x = element_text(margin = unit(c(5, 0, 0, 0), "mm")),
                  axis.title.y = element_text(margin = unit(c(0,3 , 0, 0), "mm")))

Which produces this graph:

produced graph

so as you can see, the legend to show what the black line with the points mean has not been added to my graph even though I have inputted the code. No error comes up so hence why I'm lost on whats wrong. Any ideas on what i've failed to include?

Thanks

回答1:

In order for ggplot to know to draw a legend, you need to include one of the aesthetics for a geom within aes(). In this case, if you want a legend to be drawn for your line, you need to include within the aes() in the geom_line() call one of the aesthetics that you have identified for the line: linetype or color works. We'll use color here.

Oh... and in the absence of OP sharing their dataset, here's a made-up example:

set.seed(1234)
dfMonth <- data.frame(
  Month=month.name,
  NumberMO=sample(50:380, 12),
  NumberME=sample(50:380, 12)
)

Now the code to make the plot and ensure the legend is created.

p <- ggplot(dfMonth, aes(x=Month)) +
  geom_col(aes(y=NumberMO), size=0.7, color="black", fill="white") +
  geom_line(aes(y=NumberME, color='black'), size=1, group=1)
p

We have a legend, but there's some problems. You get the default title of the legend (which is the name of the aesthetic), and the default label (which is whatever text you put inside aes(color=.... Since we put "black" as the value there, it's applied as the label, and not the actual color. The actual color of the line is to default to the first level of the standard colorset used by ggplot2, which in this case is that light red color.

To set the color, name of the legend, and name of the label, we should specify the value. There's only one item in the legend, so there's no need to specify, but if you were to send a named vector to indicate the name for our single line explicitly, you end up with the somewhat strange-looking c('black'='black'). I also included a line break in the label name to make the look a bit better. Also, the months were running into each other, so I also changed the angle of the x axis labels.

Finally, you might notice the months were out of order. That's because default ggplot2 behavior is to factor a column of discrete values, which uses alphabetical ordering for the levels. To fix that, you specify the column as a factor before plotting with the correct levels.

dfMonth$Month <- factor(dfMonth$Month, levels=month.name)

p + scale_color_manual(
        name=NULL, values=c('black'='black'),
        labels='Expected No.\nof birds') +
      theme(axis.text.x=element_text(angle=30, hjust=1))

来源：https://stackoverflow.com/questions/63384151/adding-a-legend-to-a-combined-line-and-bargraph-ggplot

标签

ggplot2

bar-chart

legend