title: Theme manipulation in ggplot2: altering x and y grid lines
body: <p>Is it possible to manipulate the spacing and size of dashed and dotted grid lines in ggplot using the themes? The following plot:</p>
  
  <pre><code>p + l + opts(panel.grid.major = theme_line(colour = 'black', linetype = 'dashed'), 
               panel.grid.minor = theme_line(colour = NA), 
               panel.background = theme_rect(colour = 'white'))
  </code></pre>
  
  <p>I'd like to change the spacing (such as in Illustrator) between dashed and dotted grid lines.</p>
  
  <p>Additionally, does anyone know if the x-axis and y-axis grid lines can be manipulated separately? For instance, I want to turn off the x-axis grid lines in this example. </p>
  
  <p>I have seen manipulation using vline and hline (<a href="https://stackoverflow.com/questions/5718871/add-a-dotted-vertical-line-on-certain-x-axis-values-using-ggplot">Add a dotted vertical line on certain x-axis values using ggplot</a>), but don't want to have to hard code that each time if possible.</p>
  
  <p><img src="https://i.stack.imgur.com/mpwhj.png" alt="dashed line"></p>