# Andy Eggers, April 2, 2008
# quick R API for google charts 
# see http://code.google.com/apis/chart/

googleplot = function(x, y, xlim = NULL, ylim = NULL, pch = 1, width = 250, height = 200){
  if (length(x) != length(y)){stop("Length of x and y differ")}
  if (is.null(xlim)){x_spread = max(x) - min(x); xlim = c(min(x) - .1*x_spread, max(x) + .1*x_spread)}   #TODO: use standard approaches for making reasonable axes, if possible
  if (is.null(ylim)){y_spread = max(y) - min(y); ylim = c(min(y) - .1*y_spread, max(y) + .1*y_spread)}
  encoded_x = simple_encode(x, xlim[1], xlim[2])
  encoded_y = simple_encode(y, ylim[1], ylim[2])
  if (length(pch) != length(x)){
    cat("repeating ", pch[1], "\n", sep = "")
    fixed_pch = rep(pch[1], length(x))
    } else {fixed_pch = pch}
  encoded_pch = simple_encode(20*fixed_pch, the_min = 0)  # TODO: rethink pch handling
  preamble = "http://chart.apis.google.com/chart?cht=s"
  chart_data = paste("chd=s:", paste(join_string(encoded_x), join_string(encoded_y), join_string(encoded_pch), sep = ","), sep = "")
  axis_declaration = "chxt=x,y" # x- and y-axis for now
  axis_labels = paste("chxl=0:|", axis_labels(xlim), "|1:|", axis_labels(ylim), sep = "")
  chart_size = paste("chs=", as.character(width), "x", as.character(height), sep = "")
  paste("<img src='", paste(preamble, chart_data, axis_declaration, axis_labels, chart_size, sep = "&amp;"), "' />", sep = "")
  }
 
simple_encode = function(vec, the_min = NULL, the_max = NULL){
  if (is.null(the_min)){the_min = min(vec)}  # take up the whole range of values if no min and max was provided
  if (is.null(the_max)){the_max = max(vec)}
  rescaled_vec = 61 * (vec - the_min)/the_max   # simple text encoding produces single character data values A-Za-z0-9
  c(LETTERS, letters, 0:9)[floor(rescaled_vec)+1]  # see http://code.google.com/apis/chart/#simple 
  }

# utility for concatenating strings from vectors to pass data to API
join_string = function(vec, sep = ""){
  out = vec[1]
  for (element in vec[2:length(vec)]){
    out = paste(out, element, sep = sep)
    }
  out
  }
  
axis_labels = function(lim_pair, length = 5){      # TODO: use standard approaches for choosing tick marks
  vec = floor(seq(from = lim_pair[1], to = lim_pair[2], length.out = length))
  join_string(vec, sep = "|")
  }