title: ggplot2 - add legend
body: <p>I have the following data:</p>
  
  <pre><code>MONTH;MEDIAN;MIN;MAX
  1;-736.12;-5272.96;5946.79
  2;3340.83;284.72;15707.46
  3;7144.85;2916.79;17659.6
  4;9927.73;1660.99;19191.29
  5;10986.7;3689.15;16474.34
  6;1526.48;-8086.48;13430.86
  7;-4415.36;-9244.11;27554.34
  8;-3213.32;-8970.76;22089.98
  9;-3435.17;-5293.95;12451.59
  10;-5112.405;-7002.67;5237.85
  11;-7820.8;-11170.12;1164
  12;-6143.055;-10836.93;165.82
  </code></pre>
  
  <p>I am plotting those data using the following code:</p>
  
  <pre><code>png("amazon_ghm.png", width = 1000, height = 400)
  ggplot(amazon_ghm) +
    geom_line(aes(MONTH, MEDIAN), group=1, size = 2, color="red4") +
    geom_ribbon(aes(MONTH, ymax = MAX, ymin = MIN), alpha = 0.5, fill = "tomato1") +
    geom_hline(aes(yintercept = 0), linetype="dotted") +
    geom_text(size=9, aes(3, 25000, label = "Upper Amazon GHM"))+
    theme_bw() +
    theme(axis.text=element_text(size=20),
          axis.title=element_text(size=20),
          axis.line = element_line(colour = "black")) +
    labs(x = "Month",
        y = "Diff in runoff [m3/s]")+
    scale_x_continuous("Month", breaks = 0:12, expand = c(0,0.05))+
    scale_colour_manual(values = c("MEDIAN"="red4", "MIN/MAX range"="tomato1"))
  graphics.off()
  </code></pre>
  
  <p>I don't know why but I can't add the legend as I want. I have tried <code>scale_colour_manual</code>, <code>geom_point</code> and a few other options but with none of them I have succeed to plot the legend.</p>
  
  <p>Any would be greatly appreciated!</p>