title: NaNs produced in scale transform
body: <p>I am trying to create a ggparcoord plot with the y-value logged of some data that has positive and negative values in it:</p>
  
  <pre><code>x = data.frame(a=2*runif(100)-1,b=2*runif(100)-1,c=2*runif(100)-1,d=2*runif(100)-1,e=2*runif(100)-1)
  dim(x)
  [1] 100   5
  </code></pre>
  
  <p>I then try to plot the parallel coordinates plot:</p>
  
  <pre><code>library(GGally)
  ggparcoord(x, columns=1:5, alphaLines=0.5) + scale_y_log10()
  </code></pre>
  
  <p>And receive the following error:</p>
  
  <pre><code>Warning messages:
  1: In scale$trans$trans(x) : NaNs produced
  2: Removed 167 rows containing missing values (geom_path).
  </code></pre>
  
  <p>I am thinking the NaNs are produced when we take a log of a negative value. However, I do not understand why 167 rows containing missing values, when the dimension of x was 100 rows.</p>
  
  <p>In any case, I try to solve this by simply adding a value of 2 to every index in x (so that values in x are now between +1 and +3).</p>
  
  <pre><code>x=x+2
  ggparcoord(x, columns=1:5, alphaLines=0.5) + scale_y_log10()
  Warning messages:
    1: In scale$trans$trans(x) : NaNs produced
  2: Removed 167 rows containing missing values (geom_path).
  </code></pre>
  
  <p>However, I receive the same message. Any idea how to solve this?</p>