title: Cannot open PDF's created in a loop in R
body: <p>I tried to generate graphs using a loop for the first time. 
  I have to generate double y-axis graphs for 400 species.
  Since they had to be double-y-axis graphs, I did not use ggplot2.
  I manage to see the graphs in Rstudio, but when I try to open the PDF (I also tried TIFF,JPEG,...) they cannot be opened. (Free translation of the error code: cannot be opened since this type of file is not supported or the file is damaged)</p>
  
  <p>I read a lot of posts on ending your code with dev.off() but this does not help. 
  this is the code. A single file generated with the same code (but without a loop) opens fine...</p>
  
  <pre><code>for (i in categories){
    plot_data&lt;-subset(af, nummer_soort_naam == i)
    par(mar = c(5,5,2,5))
    with(plot_data,
         plot(jaar,
              aantal_waargenomen_individuen_per_duizend_daghokbezoeken, 
              type="l"))# pch=16))}
    par(new = T)
    with(plot_data, 
         plot(jaar, ruwe_aantal_ind_waargenomen, 
              pch=16, axes=F, xlab=NA, ylab=NA, cex=1.2,
              main=paste(i,sep=" ")))
    axis(side = 4)
    mtext(side = 4, line = 3, 'ruwe aantal individuen waargenomen')
    legend("topleft", 
           legend=c("gecorrigeerd aantal","ruwe aantallen"),
           lty=c(1,0), 
           pch=c(NA, 16), 
           col=c("black", "black"))
    dev.print(file=paste(i,".pdf",sep=""))
    dev.off()
  }
  </code></pre>
  
  <p>The data looks like this:</p>
  
  <pre><code>soort_id nummer_soort_naam jaar ruwe_aantal_ind_waargenomen aantal_waargenomen_individuen_per_duizend_daghokbezoeken
  1 1_Roerdomp_Botaurus_stellaris 1990 6 3.84
  1 1_Roerdomp_Botaurus_stellaris 1991 12 7.89
  1 1_Roerdomp_Botaurus_stellaris 1992 10 6.9
  1 1_Roerdomp_Botaurus_stellaris 1993 7 3.7
  1 1_Roerdomp_Botaurus_stellaris 1994 37 20.19
  2 2_Dodaars_Tachybaptus_ruficollis 1990 399 248.6
  2 2_Dodaars_Tachybaptus_ruficollis 1991 569 365.9
  2 2_Dodaars_Tachybaptus_ruficollis 1992 560 376.1
  2 2_Dodaars_Tachybaptus_ruficollis 1993 312 163
  2 2_Dodaars_Tachybaptus_ruficollis 1994 373 198.3
  </code></pre>
  
  <p>Can somebody help me based on this code and see the error? </p>
  
  <p>Thank you very much in advance!</p>
  
  <p>Kristijn</p>