Drawing the protein assay with ggplot

I have been persuaded by Steph Locke during todays meeting of the Cardiff R Users Group that I should use ggplot for all the plots during our R for Biochemist Training Day. For that reason, I have written the script below which uses ggplot to graph the protein assay:

# START of SCRIPT
library(ggplot2)

# Protein Concentrations
prot <- c(0.000, 0.016, 0.031, 0.063, 0.125, 0.250, 0.500, 1.000, 
          0.000, 0.016, 0.031, 0.063, 0.125, 0.250, 0.500, 1.000) 

# Absorbance from my protein assay
abs <- c(0.329, 0.352, 0.349, 0.379, 0.417, 0.491, 0.668, 0.956, 
         0.327, 0.341, 0.355, 0.383, 0.417, 0.446, 0.655, 0.905)

# Convert into data.frame to plot with ggplot
data <- as.data.frame(prot)
data$abs <- abs

#Calculate the line using the linear model function
line <- lm(abs~prot)

#Equation of a line y = mx + c
#In our case abs = slope * prot + intercept
# ukn.prot = (abs - intercept)/slope
int <- summary(line)$coefficients[1]
slope <- summary(line)$coefficients[2]

#now calculate some unknown protein concs from absorbances
#put the unknowns into a vector
abs.ukns <- c(0.554, 0.568, 0.705)

#rearrange the equation of the line to ukn.prot = (abs - intercept)/slope
prot.ukns <- (abs.ukns - int)/slope

# create the object with the graph in it. 
p <- ggplot(data=data,          # specify the data frame with data
        aes(x=prot, y=abs)) +   # specify the x and y for the graph
        geom_point() +          # make a scatter plot
        stat_smooth(method = "lm") +  # add a linear model line
        xlab("[Protein] (microg/ml)") +   # label x-axis
        ylab("Absorbance (570nm)") +    # label y-axis
        ggtitle("Protein Assay 20th April 2015") +  # add a title
        theme_bw() +      # a simple theme
        expand_limits(y=c(0.25,1)) +    # customise the y-axis
        annotate(geom="text", x=0.85, y= 0.6, label="Abs         Prot",  color="red")

#put the answers on the graph
for (i in 1:length(abs.ukns)){
  p <- p + annotate(geom="text", x = 0.8, y = (0.6 - i/20), label=abs.ukns[i])
  p <- p + annotate(geom="text", x = 0.92, y = (0.6 - i/20), label=round(prot.ukns[i], 3))
}

p # show us the graph...

# END OF SCRIPT