imageName =
function(city, full = FALSE, mini = FALSE, ext = "png") {
  id = gsub("[ ,]", "_", city)

  if(is.logical(full)) {
     if(full)
       id = paste("file://", getwd(), id, sep = .Platform$file.sep)
  } else
       id = paste(full, id, sep = .Platform$file.sep)      

  if(mini)
    id = paste(id, "mini", sep = "_")

  id = paste(id, ext, sep = ".")
  id
}

makeHist =
function(z, device = png, ylim = range(z$temp), full = FALSE, mini = FALSE,
          cols = temperatureColors)
{
#  cols = rainbow(length(levels(z$level)))
  id = imageName(z[1, "city"], full, mini = mini)
  if(!is.null(device)) {
    device(id, width = 300, height = 300, bg = "transparent")
    on.exit(dev.off())
  }

  plot(z$temp, xlab = "", ylab = "", axes = FALSE, ylim = ylim,
          col = cols[z$level])
  segments(1:3, z$temp[1:3], 2:4, z$temp[2:4], col = cols[z$level], lwd = if(mini) 2 else 30)
  axis(2)

  id
}  

createHist =
function(data = temperature, f = makeHist, full = "/tmp", mini = FALSE)
{
  by(data, factor(data$city), f, ylim = range(data$temp), full = full, mini = mini)
}  

createLegend =
function(values, filename = "/tmp/legend.png", colors = temperatureColors)
{
  key.width = 8

  levels = values$level
  
    if(length(filename) && nchar(filename) > 0) {
       trellis.device(png, filename = filename, bg = "transparent")
       on.exit(dev.off())
     }

  trellis.par.set(axis.text = list(col = "white", cex=1.2, font=2))
#    print(simpleKey(levels(levels?dra), rectangles = list(TRUE, col = rainbow(4)))
  breaks = c(min(values$temp), by(values$temp, values$level, max))
  k = list(at = breaks, 
           labels = as.character(breaks),
           col= colors,
           width= key.width
           )
  draw.colorkey(key = k, draw = TRUE, viewport(height = unit(.9, "npc")))
}