I'm writing a function to produce time series plots of stock prices. However, I'm getting the following error

"Error in df[, 7] : object of type 'closure' is not subsettable"

Here's an example of the function:

plot.prices <- function(df) {
  require(ggplot2)
  g <- ggplot(df, aes(x= as.Date(Date, format= "%Y-%m-%d"),
              y= df[, 7])) + geom_point(size=1)

  # ... code not shown...
  g
}

And example data:

spy <- read.csv(file= 'http://ichart.finance.yahoo.com/table.csv?s=SPY&d=11&e=1&f=2012&g=d&a=0&b=29&c=1993&ignore=.csv', header= T)

plot.prices(spy) # produces error
ggplot(spy, aes(x= as.Date(Date, format= "%Y-%m-%d"),
                  y= spy[, 7])) + geom_point(size=1) 
## does not produce error

As you can see, the code is identical. I get an error if the call to ggplot() is INSIDE the function but not if the call to ggplot() is OUTSIDE the function.

Anyone have any idea why the seeming contradiction?