span> <span class='kw'>=</span> <span class='kw'>NULL</span>,
  <span class='kw'>x_na</span> <span class='kw'>=</span> <span class='fl'>TRUE</span>,
  <span class='kw'>x_pretty_n</span> <span class='kw'>=</span> <span class='fl'>6</span>,
  <span class='kw'>x_rev</span> <span class='kw'>=</span> <span class='fl'>FALSE</span>,
  <span class='kw'>x_title</span> <span class='kw'>=</span> <span class='kw'>NULL</span>,
  <span class='kw'>x_title_wrap</span> <span class='kw'>=</span> <span class='fl'>50</span>,
 