Modify legend in facetted plot, ggplot2

Question

For this graph, I want to split the legend into 2 separate legends. One showing the Pulsed Wetlands as circles with the shades corresponding to Wetland number and one showing t

Answer 1

Here is a solution inspired by this question

First, made new variable colvec2 that contains all colors only once.

colvec2 <-c("white", "gray80", "gray60", "gray37",  "black")  
colvec <-c("white", "white","gray80", "gray80", "gray60", "gray60", "gray37", "gray37","black", "black")  

Original plot saved as object and without the legend.

p<-ggplot(water, aes(Date, Temp, group=Wetland, shape=Hydrology)) +
  geom_point(aes(color=Wetland),size=3) +
  scale_colour_manual(values=colvec) +
  facet_grid(Layer ~ Hydrology) +
  theme(legend.position="none")

Plot that contains only data of Pulsed. Set shape=16 inside the geom_point(). With scale_colour_manual() changed legend title and colors.

p1<-ggplot(subset(water,Hydrology=="Pulsed"), 
     aes(Date, Temp, group=Wetland, shape=Hydrology)) +
  geom_point(aes(color=Wetland),size=3,shape=16) +
  scale_colour_manual("Pulsed Wetlands",values=colvec2) +
  facet_grid(Layer ~ Hydrology)

The same as p1, only for Static.

p2<-ggplot(subset(water,Hydrology=="Static"), 
      aes(Date, Temp, group=Wetland, shape=Hydrology)) +
  geom_point(aes(color=Wetland),size=3,shape=17) +
  scale_colour_manual("Static Wetlands",values=colvec2) +
  facet_grid(Layer ~ Hydrology)

Save plots p1 and p2 using ggplot_build() and ggplot_gtable().

pt1<-ggplot_gtable(ggplot_build(p1))
pt2<-ggplot_gtable(ggplot_build(p2))

Extract just grob of legends for both plots. This time legends are in 12. element (element containing guide-box)

leg1<-pt1$grobs[[12]]
leg2<-pt2$grobs[[12]]

Now use grid.arrange() and arrangeGrob() to plot all parts.

library(gridExtra)
grid.arrange(arrangeGrob(p,arrangeGrob(leg1,leg2), ncol = 2,widths=c(3/4,1/4)))