title: Plot - Invalid xlim value
body: <p>I want to plot two vectors in a simple line plot and then want to add a line at the 0 x axe point.</p>
  
  <p>This is how I tried to implement it:</p>
  
  <pre><code>sale &lt;- 
  structure(c(-0.049668136, 0.023675638, -0.032249731, -0.071487224, 
  -0.034017265, -0.031278933, -0.052070721, -0.034305542, -0.019041209, 
  -0.050459175, -0.017315808, -0.012787003, -0.03341208, -0.045078144, 
  -0.036638132, -0.036533367, -0.012683656, -0.014388251, -0.006775188, 
  -0.037153807, -0.008941402, -0.011760677, -0.005077979, -0.041187417, 
  -0.001966554, -0.028822067, 0.021828558, 0.016208791, -0.026897492, 
  -0.032107207, -0.008496522, -0.028027096, -0.013746662, -0.004545603, 
  -0.005679941, -0.004614187, 0.004083014, -0.012624954, -0.016362079, 
  -0.006350167, -0.019551277), na.action = structure(42:45, class = "omit"))
  
  purchase &lt;- 
  structure(c(0.042141187, 0.075875128, 0.090953485, 0.050951625, 
  0.082566915, 0.184396833, 0.136625887, 0.042725409, 0.135028692, 
  0.13201904, 0.093634104, 0.16776844, 0.13645719, 0.201365036, 
  0.227589832, 0.236473792, 0.269064385, 0.200981722, 0.144739536, 
  0.145256493, 0.040205545, 0.031577107, 0.014767345, 0.005843065, 
  0.034805051, 0.082493053, 0.010572227, 0.000645763, 0.033368236, 
  0.024326153, 0.038601182, 0.025446045, 0.000556418, 0.017201608, 
  0.008316872, 0.059722053, 0.059695415, 0.076940829, 0.067650014, 
  0.002029566, 0.008466334), na.action = structure(42:45, class = "omit"))
  
  
  timeLine &lt;- c(-20:+20)
  plot(sale,type="b", xlim=timeLine)
  
  Error in plot.window(...) : invalid 'xlim' value
  par(new=T)
  
  plot(purchase,type="b", xlim=timeLine)
  
  Error in plot.window(...) : invalid 'xlim' value
  par(new=F)
  </code></pre>
  
  <p>However I get <code>Error in plot.window(...) : invalid 'xlim' value</code>. Why is my timeLine false implemented?</p>
  
  <p>I really appreciate your answer!</p>