title: Boxcox() transformation in R error
body: <pre><code>cellphone = read.csv("/Users/crystalchau/Desktop/UICT-CELL_IND.csv", nrows = 25, colClasses = c(NA,NA,"NULL")) 
  
  cellphone = cellphone[nrow(cellphone):1,]
  
  cellphone.ts = ts(cellphone, frequency = 1)
  
  ts.plot(cellphone.ts, ylab = "Mobile Cellular Telephone Subscriptions")
  
  title(expression(Mobile~Celluar~Telephone~Subscriptions))
  
  par(mfrow=c(1,2))
  
  cellphone = read.csv("/Users/crystalchau/Desktop/UICT-CELL_IND.csv", nrows = 25, colClasses = c("NULL",NA,"NULL"))
  
  cellphone = cellphone[nrow(cellphone):1,]
  
  cellphone.ts = ts(cellphone, frequency = 1)
  
  acf(cellphone.ts, lag.max = 10)
  
  pacf(cellphone.ts, lag.max = 10)
  
  cellphone.ts = ts(cellphone, frequency = 12)
  
  decompose_cellphone = decompose(cellphone.ts, type = "multiplicative")
  
  plot(decompose_cellphone)
  
  library(MASS)
  
  bcTransform = boxcox(cellphone ~ as.numeric(1:length(cellphone)), lambda = seq(-1, 1, length = 10))
  
  plot(bcTransform, type = 'l', axes = FALSE)
  </code></pre>
  
  <p>It won't allow me to run the boxcox transformation line and gives me the error message: </p>
  
  <blockquote>
    <p>Error in boxcox.default(cellphone.ts ~
    as.numeric(1:length(cellphone.ts)),  :    response variable must be
    positive</p>
  </blockquote>
  
  <p>What am I doing wrong?</p>