title: Remove fill around legend key in ggplot
body: <p>I would like to remove the gray rectangle around the legend. I have tried various methods but none have worked.</p>
  
  <pre><code>ggtheme &lt;- 
  theme(
  axis.text.x = element_text(colour='black'),
  axis.text.y = element_text(colour='black'),
  panel.background = element_blank(),
  panel.grid.minor = element_blank(),
  panel.grid.major = element_blank(),
  panel.border = element_rect(colour='black', fill=NA),
  strip.background = element_blank(),
  legend.justification = c(0, 1),
  legend.position = c(0, 1),
  legend.background = element_rect(colour = NA),
  legend.key = element_rect(colour = "white", fill = NA),
  legend.title = element_blank()
  )
  
  colors &lt;- c("red", "blue")
  df &lt;- data.frame(year = c(1:10), value = c(10:19), gender = rep(c("male","female"),each=5))
  ggplot(df, aes(x = year, y = value)) + geom_point(aes(colour=gender))  +
  stat_smooth(method = "loess", formula = y ~ x, level=0, size = 1, 
      aes(group = gender, colour=gender)) +
  ggtheme + scale_color_manual(values = colors) 
  </code></pre>
  
  <p><a href="https://i.stack.imgur.com/PLOFd.png" rel="noreferrer"><img src="https://i.stack.imgur.com/PLOFd.png" alt="enter image description here"></a></p>