title: dashed line in ggplot legend
body: <p>I want to set the linetype of my legend. </p>
  
  <p>My data is like: </p>
  
  <pre><code>  VisitMonth VisitYear CaixaForum MNAC   FirstDay
    1:         01      2007         NA 7125 2007-01-01
    2:         02      2007         NA 5345 2007-02-01
    3:         03      2007         NA 4996 2007-03-01
    4:         04      2007         NA 5476 2007-04-01
    5:         05      2007         NA 6160 2007-05-01
   ---                                                
   98:         02      2015      17903 2360 2015-02-01
   99:         03      2015      30400 2930 2015-03-01
  100:         04      2015      25422 3088 2015-04-01
  101:         05      2015      10787 2130 2015-05-01
  102:         06      2015       3679 2047 2015-06-01
  </code></pre>
  
  <p>I want to plot the time serie of CaixaForum and MNAC column. I have code the following:</p>
  
  <pre><code>ggplot(data = MUSEUMS, aes(x = FirstDay, y = MNAC)) + 
    geom_line(size=0.75, aes(x = FirstDay, y = MNAC, colour = "MNAC")) + 
    geom_line(size=0.75, aes(y = CaixaForum, colour = "CaixaForum"), linetype = "dashed") + 
    labs(title = "", x = "", y = "Monthly Visitors") +  theme_bw() + 
    theme(legend.title = element_text(size=16, face="bold"),  legend.direction = "horizontal",
          legend.position=c(0.5, 1.05), text = element_text(size=20)) + 
    scale_colour_manual(name="Legend",values=c(MNAC="black", CaixaForum="black")) 
  </code></pre>
  
  <p>As you can observe, you can not distinguish between the two linetypes in the legend: </p>
  
  <p><img src="https://i.stack.imgur.com/gsoHg.jpg" alt="enter image description here"></p>
  
  <p>How can I fixed?</p>
  
  <p>I coded other answers in stackoverflow but they haven't worked.</p>