我们可以使用参数 show.legend 从 geom_sf 这里。

  ggplot（）+ 
 geom_sf（data = simple.lines.sf，aes（color = as.factor（ 显示的描述show .legend  from ？geom_sf  
 
逻辑。该图层是否应该包含在图例中？ NA（默认值）包括是否映射任何美学。 FALSE从不包含，并且TRUE始终包含。 
您也可以将其设置为多边形，线条和点中的一个以覆盖默认图例。 


 

I have a couple spatial shape files with various public transport routes and I would like to make a map using ggplot2 and sf libraries. The issue here is that I manually assign colors to a few specific routes but I couldn't manage to add a legend to the plot. 

Any idea on how to do this using geom_sf ?

Reproducible example
library(sf)
library(ggplot2)

# reproducible data
  lon<-c(5.121420, 6.566502, 4.895168, 7.626135)
  lat<-c(52.09074, 53.21938, 52.37022, 51.96066)
  cities<-c('utrecht','groningen','amsterdam','munster')
  size<-c(300,500,1000,50)

  xy.cities<-data.frame(lon,lat,cities,size)

  # line example
  line1 <- st_linestring(as.matrix(xy.cities[1:2,1:2]))
  line2 <- st_linestring(as.matrix(xy.cities[3:4,1:2]))

  lines.sfc <- st_sfc(list(line1,line2))
  simple.lines.sf <- st_sf(id=1:2,size=c(10,50),geometry=lines.sfc)

# plot
  ggplot() + 
    geom_sf(data= subset(simple.lines.sf, id==1), color="red" ) +
    geom_sf(data= subset(simple.lines.sf, id==2), color="blue" )



I know it would be possible to do something like this:
  ggplot() + 
    geom_sf(data= subset(simple.lines.sf, id>0), aes(color=factor(id)) ) +
    scale_color_manual(values=c("red", "blue"), 
                       labels=c("route 1", "route 2"))



However, I'm working with more than one shape file so I need to use more than one geom_sf. Also, I would like the legend to look a line legend, not a polygon legend.

We can use the argument show.legend from geom_sf here. 
ggplot() + 
 geom_sf(data= simple.lines.sf, aes(colour = as.factor(id)), show.legend = "line")

Description of show.legend from ?geom_sf 

  
logical. Should this layer be included in the legends? NA, the default, includes if any aesthetics are mapped. FALSE never includes, and TRUE always includes.
  You can also set this to one of "polygon", "line", and "point" to override the default legend.