title: plot monthly data with xyplot with R
body: <p>I want to plot this data frame with month on x-axis. </p>
  
  <pre><code>month  value1  value2  value3  value4
  1   Okt 19.5505 19.6145 19.5925 19.3710
  2   Nov 21.8750 21.7815 21.7995 20.5445
  3   Dez 25.4335 25.2230 25.2800 22.7500
  
  t = read.csv("Mappe1.csv", header = TRUE, sep=";", dec = ".", fill = TRUE, comment.char = "")
  
  t$m &lt;- factor(t$m, levels = c("Okt", "Nov", "Dez"))
  
  library(Hmisc)
  
  xyplot(t$value1~t$m, type = "l", col = "red", ylab="values")
  lines(t$value2~t$m, type = "l", col = "cyan")
  lines(t$value3~t$m, type = "l", col = "purple")
  lines(t$value4~t$m, type = "l", col = "black", lwd = 2)
  legend("topleft", legend=c("value1", "value2", "value3", "value4"),
     col=c("red", "cyan", "purple", "black"), lty=1:1, cex=0.8)
  </code></pre>
  
  <p>it worked out very well for this example. but when I tried it exactely the same way but with different values, only value1 is plottet and I always get the following errors:</p>
  
  <pre><code>Error in plot.xy(xy.coords(x, y), type = type, ...) : 
    plot.new has not been called yet
  Error in strwidth(legend, units = "user", cex = cex, font = text.font) : 
    plot.new has not been called yet
  </code></pre>
  
  <p>I already applied plot.new() and dev.off(). But sometimes I still get these errors or sometimes R doesn't show errors but doesn't plot at all.</p>
  
  <p>What might be the problem here? </p>
  
  <p>Many thanks in advance for your help!</p>