title: ggplot2: how to remove slash from geom_density legend
body: <p>I'm trying to plot some overlapping density plots in ggplot2. I'm running into a problem where I cannot remove the diagonal slash from the legend. I have tried using scale_fill_manual() and legend.key as well as the hack from R Cookbook, but I cant seem to get it right. </p>
  
  <pre><code>data(iris)
  iris=iris
  cols=brewer.pal(3,"Set1")
  
  ggplot(iris) + 
      geom_density(position="identity",aes(x=iris$Sepal.Length,fill=cols[1]),
          colour="black",alpha=.5) +
      geom_density(position="identity",aes(x=iris$Sepal.Width,fill=cols[2]),
          colour="black",alpha=.5)+  
      theme_bw() +
      scale_fill_identity(guide="legend",labels=c("Sepal Width","Sepal Length"))+
      xlab("X axis") +
      theme(panel.background=element_blank(),
          legend.title=element_blank(),
          legend.key = element_rect(),
          legend.background = element_blank(),
          legend.justification=c(1,0), 
          legend.position=c(.75,.5),
          panel.grid.major = element_blank(),
          panel.grid.minor = element_blank(),
          panel.border = element_blank(),
          panel.background = element_blank())
  </code></pre>
  
  <p><img src="https://i.stack.imgur.com/00mAX.jpg" alt="enter image description here"></p>
  
  <p>What can I do to solve this? </p>