Trying to remove margins on ggplot2 visualization

Question

The function scale_y_continuous(expand=c(0,0)) is giving me the error:

Error: Discrete value supplied to continuous scale

I really don't understand what this means. the function is called scale_y_continuous, not scale_y_discrete. Here's my data.

require(ggplot2)  # ggplot
gp <- read.csv("Retail_Gas_Prices.csv")
gp$Date <- as.Date(substr(gp$Date, 1, 10), "%m/%d/%Y")
gp_melted <- melt(gp, id = "Date")

gas_ml_plot <- ggplot(subset(gp_melted, variable != "Weekly.US"), 
               aes(Date, value, colour = variable)) + 
               geom_line() + ggtitle("Retail Gas Prices In The US") + 
               theme(axis.title.x = element_blank()) +
               ylab("Cost in Dollars") + 
               theme(axis.ticks = element_blank()) + 
               labs(colour = "US Region") + 
               scale_color_discrete(labels = c("East Coast", "Midwest", 
               "Gulf Coast", "Rocky Mountain", "West Coast")) + 
               theme(legend.background = element_blank()) + 
               theme(legend.position = c(0, 1)) + 
               theme(legend.justification = c(0, 1)) + 
               scale_y_continuous(expand = c(0, 0)) + 
               scale_x_discrete(expand = c(0, 0))

Answer 1

The error message is wrong in that the problem is that a Date object was passed to a discrete x scale, not that a discrete value was provided to a continuous scale. The solution is to use the correct x scale; since the x variable is a Date, use scale_x_date(expand = c(0, 0)) in place of scale_x_discrete(expand = c(0, 0)).

I have submitted a bug report describing this problem at https://github.com/hadley/ggplot2/issues/783