title: plot.ts with years at x-axis
body: <p>I looked everywhere but still could not figure out my little problem.
  I have a data frame like this:</p>
  
  <pre><code>     GerProd_sum_PerYear
  1997           369332000
  1998           399127000
  1999           396103500
  2000           506698500
  2001           417757000
  2002           440025882
  2003           499654816
  2004           533781000
  2005           565508000
  2006           600001000
  2007           695574663
  2008           543780271
  2009           496257990
  2010           547352965
  2011           554533553
  2012           532066522
  2013           535117263
  </code></pre>
  
  <p>I would like to plot.ts (or just plot), so that the years (1997 to 2013) are at the x-axis.</p>
  
  <p>So far I have done this:</p>
  
  <pre><code>test&lt;-plot.ts(df, type= "b", main = "Amounts over time", xlab = "Years", las=3, ylab = "Amounts per year")
  </code></pre>
  
  <p>It looks fine BUT the x achsis is 1 to 17, as there is 17 values.... I would like the 1 to 17 replaced by 1997 to 2013.</p>
  
  <p>Help :)</p>