Drawing multiple ablines per panel with facet_grid in R

I have a 2x2 grid graph with two models' data per panel. I need to have separate regression lines for each model in each panel. Can I do this with facet_grid? I have everything but the regression lines:

df <- data.frame(year=factor(c(2000,2001,2002,2000,2001,2002,2000,2001,2002,2000,2001,2002,
                        2000,2001,2002,2000,2001,2002,2000,2001,2002,2000,2001,2002)),
                 crop=c("Maize","Maize","Maize","Maize","Maize","Maize",
                        "Soybean","Soybean","Soybean","Soybean","Soybean","Soybean",
                        "Maize","Maize","Maize","Maize","Maize","Maize",
                        "Soybean","Soybean","Soybean","Soybean","Soybean","Soybean"),
                 treatment_code=c("T1","T1","T1","T2","T2","T2",
                                  "T1","T1","T1","T2","T2","T2",
                                  "T1","T1","T1","T2","T2","T2",
                                  "T1","T1","T1","T2","T2","T2"),
                 value=c(9,10,8.5,11,12,7.8,
                         4,7,7.5,3,3,5,
                         12,11,9,13,9,10,
                         5,3,6,4,5,4),
                 model=c("A","A","A","A","A","A","A","A","A","A","A","A",
                         "B","B","B","B","B","B","B","B","B","B","B","B"))

gY_calib <- df %>%
  ggplot(aes(x=year, y=value, color=model, show.legend=TRUE)) +
  geom_point(show.legend=TRUE) +
  xlab("Year") +
  ylab(expression('Grain Yield (Mg ha ' ^-1*')')) +
  ylim(0,13) +
  facet_grid(crop~treatment_code) 

gY_calib

Solution

Your year is factor. If you make it numeric it will work. You can use the following code

df <- data.frame(year=c(2000,2001,2002,2000,2001,2002,2000,2001,2002,2000,2001,2002,
                               2000,2001,2002,2000,2001,2002,2000,2001,2002,2000,2001,2002),
                 crop=c("Maize","Maize","Maize","Maize","Maize","Maize",
                        "Soybean","Soybean","Soybean","Soybean","Soybean","Soybean",
                        "Maize","Maize","Maize","Maize","Maize","Maize",
                        "Soybean","Soybean","Soybean","Soybean","Soybean","Soybean"),
                 treatment_code=c("T1","T1","T1","T2","T2","T2",
                                  "T1","T1","T1","T2","T2","T2",
                                  "T1","T1","T1","T2","T2","T2",
                                  "T1","T1","T1","T2","T2","T2"),
                 value=c(9,10,8.5,11,12,7.8,
                         4,7,7.5,3,3,5,
                         12,11,9,13,9,10,
                         5,3,6,4,5,4),
                 model=c("A","A","A","A","A","A","A","A","A","A","A","A",
                         "B","B","B","B","B","B","B","B","B","B","B","B"))

ggplot(df, aes(x=year, y=value)) +
  geom_point(stat='identity', position='identity', aes(color=model)) +
  xlab("Year") +
  ylab(expression('Grain Yield (Mg ha ' ^-1*')')) +
  ylim(0,13) + 
  geom_smooth(method='lm', formula= y~x, aes(colour = model)) +
  facet_grid(crop~treatment_code) +
  scale_x_continuous(labels = scales::label_number(accuracy = 1), n.breaks = 3)