Adding a legend to a ggplot with facet_wrap

Question

I am using facet-wrap to show two variables (a and b) for four cities. I could group the plots based on the cities but can not show the legend for the variables (i.e., a and b) using scale_color_discrete.

ggplot() + 
   geom_line(data=df, aes(x=year, y = a, group=city), colour="red",linetype = "longdash",show_guide = TRUE) + 
   geom_line(data=df, aes(x=year, y = b, group=city), colour="blue", show_guide = TRUE) +
   scale_color_discrete(name="Scenarios",labels=c("a" ,"b")) +
   guides(color=guide_legend(ncol=2)) +
   theme(legend.position="bottom")  +
   facet_wrap( ~ city, ncol=2) 

Here is a subset of my data:

structure(list(year = c(2015, 2016, 2016, 2016, 2016, 2016, 2016, 
2016, 2016, 2017, 2017, 2017, 2017, 2017, 2017, 2017, 2017, 2018, 
2018, 2018), 
          city = c("Calgary", "Calgary", "Calgary", "Halifax", 
"Halifax", "Ottawa", "Ottawa", "Yellowknife", "Yellowknife", 
"Calgary", "Calgary", "Halifax", "Halifax", "Ottawa", "Ottawa", 
"Yellowknife", "Yellowknife", "Calgary", "Calgary", "Halifax"), 
          a = c(25988.04, 37842.33, 37842.33, 11595.46, 11595.46, 49458.24, 
49458.24, 185.31, 185.31, 36718.9, 36718.9, 11176.82, 11176.82, 
47606.45, 47606.45, 176.5, 176.5, 36303.91, 36303.91, 10976.56),        
          b = c(25988.04, 37842.33, 37842.33, 11595.46, 11595.46, 49458.24, 
49458.24, 185.31, 185.31, 36718.9, 36718.9, 11176.82, 11176.82, 
47606.45, 47606.45, 176.5, 176.5, 36303.91, 36303.91, 10976.56
)), row.names = c(NA, -20L), 
          class = c("grouped_df", "tbl_df", 
"tbl", "data.frame"), vars = c("year", "city"), drop = TRUE, indices = list(
    0L, 1:2, 3:4, 5:6, 7:8, 9:10, 11:12, 13:14, 15:16, 17:18, 
    19L), 
          group_sizes = c(1L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 
2L, 1L), 
          biggest_group_size = 2L, labels = structure(list(year = c(2015, 
2016, 2016, 2016, 2016, 2017, 2017, 2017, 2017, 2018, 2018), 
           city = c("Calgary", "Calgary", "Halifax", "Ottawa", "Yellowknife", 
    "Calgary", "Halifax", "Ottawa", "Yellowknife", "Calgary", 
    "Halifax")), row.names = c(NA, -11L), class = "data.frame", vars = c("year", 
"city"), drop = TRUE))

Answer 1

You can assign colors to variables and then use scale_colour_manual to do it, as follows:

vars <- c("a"="red", "b"="blue")
ggplot() + 
  geom_line(data=df, aes(x=year, y = a, colour="a"), linetype = "longdash") + 
  geom_line(data=df, aes(x=year, y = b, colour="b")) +
  scale_colour_manual(name="Scenarios:", values=vars) +
  theme(legend.position="bottom")  +
  facet_wrap( ~ city, ncol=2) 

Hope it helps.