Plotting bold LETTERS with ggplot2?

Question

I'm using ggplot2 to construct a graphic with four plots, each composed of a particular modeling domain. I want to make the following plot with the letters on the plot in bold. Using symbols is too problematic, since I have 8 categories and the symbols look too similar.

Below is a reproducible example.

Data:

Covariate.Weights.Data= structure(list(Species = structure(c(1L, 1L, 1L, 1L, 2L, 2L, 
2L, 2L, 3L, 3L, 3L, 3L, 4L, 4L, 4L, 4L, 5L, 5L, 5L, 5L, 6L, 6L, 
6L, 6L, 7L, 7L, 7L, 7L, 8L, 8L, 8L, 8L, 1L, 1L, 1L, 1L, 2L, 2L, 
2L, 2L, 3L, 3L, 3L, 3L, 4L, 4L, 4L, 4L, 5L, 5L, 5L, 5L, 6L, 6L, 
6L, 6L, 7L, 7L, 7L, 7L, 8L, 8L, 8L, 8L, 1L, 1L, 1L, 1L, 2L, 2L, 
2L, 2L, 3L, 3L, 3L, 3L, 4L, 4L, 4L, 4L, 5L, 5L, 5L, 5L, 6L, 6L, 
6L, 6L, 7L, 7L, 7L, 7L, 8L, 8L, 8L, 8L), .Label = c("American black duck", 
"American wigeon", "Blue-winged teal", "Gadwall", "Green-winged teal", 
"Mallard", "Northern pintail", "Northern shoveler"), class = "factor"), 
    Flyway = structure(c(1L, 2L, 1L, 2L, 1L, 2L, 1L, 2L, 1L, 
    2L, 1L, 2L, 1L, 2L, 1L, 2L, 1L, 2L, 1L, 2L, 1L, 2L, 1L, 2L, 
    1L, 2L, 1L, 2L, 1L, 2L, 1L, 2L, 1L, 2L, 1L, 2L, 1L, 2L, 1L, 
    2L, 1L, 2L, 1L, 2L, 1L, 2L, 1L, 2L, 1L, 2L, 1L, 2L, 1L, 2L, 
    1L, 2L, 1L, 2L, 1L, 2L, 1L, 2L, 1L, 2L, 1L, 2L, 1L, 2L, 1L, 
    2L, 1L, 2L, 1L, 2L, 1L, 2L, 1L, 2L, 1L, 2L, 1L, 2L, 1L, 2L, 
    1L, 2L, 1L, 2L, 1L, 2L, 1L, 2L, 1L, 2L, 1L, 2L), .Label = c("Atlantic", 
    "Mississippi"), class = "factor"), Season = structure(c(1L, 
    1L, 2L, 2L, 1L, 1L, 2L, 2L, 1L, 1L, 2L, 2L, 1L, 1L, 2L, 2L, 
    1L, 1L, 2L, 2L, 1L, 1L, 2L, 2L, 1L, 1L, 2L, 2L, 1L, 1L, 2L, 
    2L, 1L, 1L, 2L, 2L, 1L, 1L, 2L, 2L, 1L, 1L, 2L, 2L, 1L, 1L, 
    2L, 2L, 1L, 1L, 2L, 2L, 1L, 1L, 2L, 2L, 1L, 1L, 2L, 2L, 1L, 
    1L, 2L, 2L, 1L, 1L, 2L, 2L, 1L, 1L, 2L, 2L, 1L, 1L, 2L, 2L, 
    1L, 1L, 2L, 2L, 1L, 1L, 2L, 2L, 1L, 1L, 2L, 2L, 1L, 1L, 2L, 
    2L, 1L, 1L, 2L, 2L), .Label = c("Fall", "Spring"), class = "factor"), 
    Covariates = structure(c(1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 
    1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 
    1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 2L, 2L, 2L, 2L, 2L, 2L, 
    2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 
    2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 3L, 3L, 3L, 3L, 
    3L, 3L, 3L, 3L, 3L, 3L, 3L, 3L, 3L, 3L, 3L, 3L, 3L, 3L, 3L, 
    3L, 3L, 3L, 3L, 3L, 3L, 3L, 3L, 3L, 3L, 3L, 3L, 3L), .Label = c("Forage", 
    "Habitat", "Survey"), class = "factor"), Weights = c(0.31, 
    1, 0, 0.96, 0, 0, 1, 0, 1, 1, 0, 0.06, 1, 1, 1, 0.06, 0, 
    1, 0.01, 1, 1, 1, 1, 1, 0, 1, 0, 0, 0, 0.82, 1, 1, 0.31, 
    0.43, 0, 0.82, 1, 1, 1, 0.92, 1, 0, 1, 0.97, 1, 1, 0, 0.94, 
    1, 1, 0.05, 0, 1, 1, 1, 1, 1, 1, 1, 0.07, 0.99, 1, 1, 1, 
    0.69, 0, 1, 0.04, 0, 0, 0, 0.08, 0, 0, 0, 0.02, 0, 0, 0, 
    0, 0, 0, 0.94, 0, 0, 0, 0, 0, 0, 0, 0, 0.93, 0.01, 0, 0, 
    0)), .Names = c("Species", "Flyway", "Season", "Covariates", 
"Weights"), class = "data.frame", row.names = c(NA, -96L))

Plot:

require(ggplot2)    
Covariates.Weights.Plot = 
      ggplot(Covariate.Weights.Data, aes(x=Covariates,y=Weights,shape=Species)) +
      facet_grid(Flyway~Season) + ylab("Relative Importance") +
      geom_point(size=5, position=position_jitter(h=0.1,w=0.3)) +
      scale_shape_manual(values=LETTERS[c(1,23,2,7,20,13,16,19)]) + 
      theme(text=element_text(size=20, face="bold"), 
            axis.text.y=element_text(size=20),
            axis.text.x=element_text(size=20), 
            axis.title.y=element_text(size=30, face="bold", vjust=1),
            axis.title.x=element_text(size=30, face="bold", vjust=0),
            strip.text=element_text(size=30, colour="black", face="bold", 
                                    vjust=1))
    Covariates.Weights.Plot

Answer 1

Perhaps a ggplot expert will know if this is possible directly in ggplot, but since ggplot uses grid you can

require(grid)
grid.edit("geom_point.points", grep = TRUE, global=TRUE, gp = gpar(fontface="bold"))

to bold the letters after the fact.

Answer 2

You can plot slightly bigger letters on top. In order for them to align properly (at least easily anyways), it's better to use dodge.

  ggplot(...)+...+
    geom_point(size=5, position=position_dodge(width=.6)) +
    geom_point(size=5.8,position=position_dodge(width=.6))+
  ...