PlotMetadata = function(M, xlim = range(M$t, na.rm = TRUE)){
  ## check format of M
  if('metadata' %in% names(M)){ ## if yes, this is just the output of ReadGem, and needs to be reformatted
    M = M$metadata
    M$t = POSIXct2jd(M$t)
  }
  ## in case of data gaps, add NaNs to avoid drawing long lines through gaps
  w = which(diff(M$t) > 60/86400) # identify gaps that are longer than 1 minute
  if(length(w) > 0){
    M = as.data.frame(M)
    L = list()
    w = c(1, w, length(M$t))
    w1 = c(1, w+1)
    w2 = c(w, length(M$t))
    for(i in 1:length(w1)){
      L[[i]] = M[w1[i]:w2[i],]
    }
    M = L[[1]]
    for(i in 2:length(L)){
      M = rbind(M, NaN, L[[i]])
    }
  }
    
  t = M$t
  temp = M$temp
  batt = M$batt
  gps = M$gpsOn
  
  
  ## in case of very stable power supply, increase the battery plot range
  battrange = range(batt, na.rm = TRUE)
  battcent = sqrt(prod(battrange))
  if(battrange[2]/battcent < 1.1){
    battrange = battcent * c(1/1.1, 1.1)
  }
  
  ylim = min(temp, na.rm = TRUE) + (max(temp, na.rm = TRUE) - min(temp, na.rm = TRUE)) * c(-0.05, 1.15)
  par(mar = c(4,4,4,4))
  plot(t, temp, type = 'l', col = 'red', yaxt = 'n', main = 'Battery and Temperature', xlab = 'Time (day of year)', ylab = 'Battery (V)', ylim = ylim, xlim = xlim)
  lines(t, rescale(batt, ylim, battrange), col='black')
  labels = pretty(battrange)
  axis(2, at = rescale(labels, ylim, battrange), labels = labels, col.axis = 'black', col.ticks = 'black')
  axis(4, at = pretty(range(temp, na.rm = TRUE)), col.axis = 'red', col.ticks = 'red')
  mtext('Temperature (deg C)', 4, col = 'red', line = 2.5)
  legend(x = 'topright', pch = c(15,15,15), col = c(1,2,3), legend = c('Battery', 'Temperature', 'GPS'))
  
  lines(t, gps * 0.02*(ylim[2]-ylim[1]) + ylim[1], col = 3)
}

rescale = function(x, newlim, oldlim = range(x, na.rm = TRUE)){
  (x - oldlim[1])/(oldlim[2] - oldlim[1]) * (newlim[2] - newlim[1]) + newlim[1]
}