title: Gnuplot does not plot negative values
body: <p>Data file bipdeutschland.csv:</p>
  
  <pre><code>1991    79.09   0
  1992    80.61   1.9
  1993    79.84   –1.0
  1994    81.8    2.5
  1995    83.19   1.7
  1996    83.84   0.8
  1997    85.37   1.8
  1998    87.05   2
  1999    88.78   2
  2000    91.43   3
  2001    92.98   1.7
  2002    92.99   0
  2003    92.32   –0.7
  2004    93.41   1.2
  2005    94.07   0.7
  2006    97.56   3.7
  2007    100.75  3.3
  2008    101.81  1.1
  2009    96.07   –5.6
  2010    100.00  4.1
  2011    103.59  3.6
  2012    103.98  0.4
  2013    104.09  0.1
  2014    105.76  1.6
  </code></pre>
  
  <p>PLT file:</p>
  
  <pre><code>set xdata time
  set timefmt "%Y"
  set format x "%Y"
  set format y "%6.0f"
  set xrange ["1990":"2015"]
  set yrange [0:120]
  set style fill solid 1.0
  set boxwidth 0.85 relative
  
  set xlabel "Jahr"
  set ylabel "Reales Bruttoinlandsprodukt (2010=100%)"
  
  set y2range [-5:5]
  set y2label "Veränderung des Bruttoinlandsprodukts zum Vorjahr (in %)"
  set y2tics 2
  set ytics nomirror
  set xtics nomirror
  
  set output "bipdeutschland.png"
  set term png size 1000, 500
  plot "bipdeutschland.csv" using 1:2 title '' w boxes lc rgb "green", "bipdeutschland.csv" using 1:3 title '' with lines lw 4 lc rgb "red" axes x1y2;
  </code></pre>
  
  <p><img src="https://i.stack.imgur.com/Ek3HH.png" alt="The output of the script"></p>
  
  <p>The red line should be in the negative range for 1993, 2003 and 2009, but it clearly isn't. Where is the problem, does gnuplot fails to parse the input file or is my code wrong?</p>