add script to download and preprocess the household power data
make date selection take strings; make accessors for both raw and clean data
add downloaded/unzipped data
add png output wrapper function
add function for plot 1 (histogram of global active power)
add output for plot 1
subsetting takes too long; memoize it
combine dates and times into one timestamp
typos in strptime() format string
widen date range to actually get two days of data
make sourcing of other scripts conditional
regen
add function for plot2 (line chart of global active power)
add output for plot 2
no need to factor date range out into another variable
add function for plot 3 (line chart of 3 submetering variables)
add output for plot 3
let data be passed as an optional argument to plotting functions
let caller turn off box around plot 3's legend
add function for plot 4 (four plots in a 2x2 table)