r - Error in eval(expr, envir, enclos) - contradiction?

Question

Edited to give a fuller example of code and specific issue

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

Error in eval(expr, envir, enclos) : object 'df1234' not found

Here's an example of the function:

plot.prices <- function(df1234) {
  require(ggplot2)
  g <- ggplot(df1234, aes(x= as.Date(Date, format= "%Y-%m-%d"), y= df1234[, 3], 
              colour= brewer.pal(12,"Set3")[1])) + geom_point(size=1)
  g + geom_point(aes(x= date, y = df1234[, 4], 
                 colour= brewer.pal(12,"Set3")[2]), 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
g <- ggplot(spy, aes(x= as.Date(Date, format= "%Y-%m-%d"), y= spy[, 3], 
              colour= brewer.pal(12,"Set3")[1])) + geom_point(size=1)
  g + geom_point(aes(x= as.Date(Date), y = spy[, 4], 
                 colour= brewer.pal(12,"Set3")[2]), 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?

Answer 1

The error occur because you use df[, 7] in gglpot2, use column name Adj.Close will fix the problem.

 g <- ggplot(df, aes(x= as.Date(Date, format= "%Y-%m-%d"),
                  y= Adj.Close)) + geom_point(size=1)

In fact the error , it is a scoping error. aes can't find the df environnement. It tries to look for it the global scope .

if you you want to use use indexing calls , you can use aes_string for example , and manipulate strings not expressions

plot.prices <- function(df) {
  require(ggplot2)

  df$Date <- as.Date(df$Date, format= "%Y-%m-%d")

  g <- ggplot(df, aes_string(x= 'Date',
                      y= colnames(df)[7])) + geom_point(size=1)

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