title: ggplot2: How much do "panel.grid.major" and "panel.grid.minor" inherit from "panel.grid"?
body: <p>According to the documentation of ggplot2, found <a href="https://cran.r-project.org/web/packages/ggplot2/ggplot2.pdf" rel="nofollow noreferrer">here</a>, both theme elements "panel.grid.minor" and "panel.grid.major" inherit from "panel.grid".</p>
  
  <p>It seems that "panel.grid.major" inherits the size from "panel.grid", but not the color:</p>
  
  <pre><code>library(ggplot2)
  
  data &lt;- data.frame( x = 1:12, y = 3*sin(1:12)-exp((1:12)/5) )
  
  plt &lt;- ggplot(data) +
    geom_line( aes(x=x,y=y), size=3, color="blue" ) +
    theme( panel.background = element_rect( fill="darkgrey" ),
           axis.text = element_text( size=18 ) )
  </code></pre>
  
  <p>.</p>
  
  <pre><code>&gt; plt
  </code></pre>
  
  <p><a href="https://i.stack.imgur.com/vAi0T.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/vAi0T.png" alt="enter image description here"></a></p>
  
  <pre><code>&gt; plt + theme( panel.grid = element_line( size=3, color="black" ) )
  </code></pre>
  
  <p><a href="https://i.stack.imgur.com/mCQri.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/mCQri.png" alt="enter image description here"></a></p>
  
  <pre><code>&gt; plt + theme( panel.grid = element_line( size=3 ),
  +              panel.grid.major = element_line( color="black" ) )
  </code></pre>
  
  <p><a href="https://i.stack.imgur.com/eFG8S.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/eFG8S.png" alt="enter image description here"></a></p>
  
  <p>The minor grid lines do neither inherit the size, nor the color.</p>
  
  <p>I'm using version 2.0.0 of ggplot2:</p>
  
  <pre><code>&gt; sessionInfo()
  R version 3.2.3 (2015-12-10)
  Platform: x86_64-w64-mingw32/x64 (64-bit)
  Running under: Windows 7 x64 (build 7601) Service Pack 1
  
  locale:
  [1] LC_COLLATE=German_Germany.1252  LC_CTYPE=German_Germany.1252   
  [3] LC_MONETARY=German_Germany.1252 LC_NUMERIC=C                   
  [5] LC_TIME=German_Germany.1252    
  
  attached base packages:
  [1] stats     graphics  grDevices utils     datasets  methods   base     
  
  other attached packages:
  [1] plyr_1.8.3    igraph_1.0.1  ggplot2_2.0.0
  
  loaded via a namespace (and not attached):
  [1] labeling_0.3     colorspace_1.2-6 scales_0.3.0     magrittr_1.5    
  [5] tools_3.2.3      gtable_0.1.2     Rcpp_0.12.2      grid_3.2.3      
  [9] munsell_0.4.2   
  &gt; 
  </code></pre>
  
  <p>Is there a more detailed documentation of "theme", or is trial and error the best one can do?</p>