# Function to calculate monthly returns on a stock 
monthly_stock_returns <- function(ticker, start_year) {
  # Download the data from Yahoo finance
  symbol <- getSymbols(ticker, src = 'yahoo', auto.assign = FALSE, warnings = FALSE) 
  # Tranform it to monthly returns using the periodReturn function from quantmod
  data <- periodReturn(symbol, period = 'monthly', subset=paste(start_year, "::", sep = ""), 
                       type = 'log')
  # Let's rename the column of returns to something intuitive because the column name is what
  # will eventually be displayed on the time series graph.
  colnames(data) <- as.character(ticker)
  # We want to be able to work with the xts objects that result from this function 
  # so let's explicitly put them to the global environment with an easy to use 
  # name, the stock ticker.
  assign(ticker, data, .GlobalEnv)