Changing alpha doesn't affect anything in ggplot2 - r

Question

I'm somewhat new to R and ggplot2 so this question is likely somewhat low-level. But I've done a fair amount of experimenting and found no answers online, so I thought I'd ask here.
When I add alpha to my graph, the graph appears as follows:
Some alpha
However, no matter how I change the value of alpha, I get no changes in the graph. I tried alpha=.9 and alpha=1/10000, and there was no difference whatsoever in the graph.
Yet it seems that the 'alpha' term is doing something. When I remove the 'alpha' from the code, I get the following graph:
No alpha
Here's my code. Thanks!
library(ggplot2)
library(chron)
argv <- commandArgs(trailingOnly = TRUE)
mydata = read.csv(argv[1])
png(argv[2], height=300, width=470)
timeHMS_formatter <- function(x) { # Takes time in seconds from midnight, converts to HH:MM:SS
h <- floor(x/3600)
m <- floor(x %% 60)
s <- round(60*(x %% 1)) # Round to nearest second
lab <- sprintf('%02d:%02d', h, m, s) # Format the strings as HH:MM:SS
lab <- gsub('^00:', '', lab) # Remove leading 00: if present
lab <- gsub('^0', '', lab) # Remove leading 0 if present
}
dateEPOCH_formatter <- function (y){
epoch <- c(month=1,day=1,year=1970)
chron(floor(y),out.format="mon-year",origin.=epoch)
}
p= ggplot() +
coord_cartesian(xlim=c(min(mydata$day),max(mydata$day)), ylim=c(0,86400)) + # displays data from first email through present
scale_color_hue() +
xlab("Date") +
ylab("Time of Day") +
scale_y_continuous(label=timeHMS_formatter, breaks=seq(0, 86400, 7200)) + # adds tick marks every 2 hours
scale_x_continuous(label=dateEPOCH_formatter, breaks=seq(min(mydata$day), max(mydata$day), 365) ) +
ggtitle("Email Sending Times") + # adds graph title
theme( legend.position = "none", axis.title.x = element_text(vjust=-0.3)) +
layer(
data=mydata,
mapping=aes(x=mydata$day, y=mydata$seconds, alpha=1/2, size=5),
stat="identity",
stat_params=list(),
geom="point",
geom_params=list(),
position=position_identity(),
)
print(p)
dev.off()

Answer 1

You need to put the alpha specification outside the mapping statement, as in
layer(
data=mydata,
mapping=aes(x=day, y=seconds),
stat="identity",
stat_params=list(),
geom="point",
geom_params=list(alpha=1/2, size=5),
position=position_identity(),
)
I'm more used to expressing this somewhat more compactly as
geom_point(data=mydata,
mapping=aes(x=day, y=seconds),
alpha=1/2,size=5)
The rest of the excluded stuff represents default values, I believe ...
See also: Why does the ggplot legend show the "colour" parameter?