title: geom_segment arrow direction
body: <p>I'm trying to plot some up and down arrows using <code>geom_segment</code>. I splitted my dataset in 2 for the data that need an up-facing arrow and the down-facing ones. </p>
  
  <p>But on both plots the arrow is facing upwards. how can this be reversed? Is there a variable that i'm missing?</p>
  
  <p>The dataset I'm using looks like the following:</p>
  
  <pre><code>GENE   START    END        STRAND
  A      3000000  3000312    +
  B      3001233  3090123    -
  </code></pre>
  
  <p>The positive strands need an <code>up-facing</code> arrow, while the negative ones, need a <code>down-facing</code>.</p>
  
  <p>here's my code:</p>
  
  <pre><code># split the dataset in '+' and '-'
  up.arrows &lt;- genes.data[which(genes.data$STRAND=='+'),]
  up.arrows.x &lt;- runif(length(up.arrows$START), min = 0, max = length(up.arrows))
  
  down.arrows &lt;- genes.data[which(genes.data$STRAND=='-'),]
  down.arrows.x &lt;- runif(length(down.arrows$START), min = 0, max = length(down.arrows))
  </code></pre>
  
  <p>I'm using runif, because I don't care about the <code>x-axis</code> position. My main concern is the <code>y-axis</code>.</p>
  
  <pre><code>ggplot(up.arrows, aes(up.arrows.x, up.arrows$START)) +
      geom_segment(aes(xend=up.arrows.x, yend=up.arrows$STOP), 
                   arrow = arrow(length = unit(0.1, "cm")),
                   colour = 'green4') + 
      geom_text_repel(size = 3,
                      aes(label = up.arrows$GENE),
                      color = 'black') +
      theme_classic(base_size = 16)
  
  ggplot(down.arrows, aes(down.arrows.x, down.arrows$START)) +
      geom_segment(aes(xend=down.arrows.x, yend=down.arrows$STOP), 
                   arrow = arrow(length = unit(0.1, "cm")),
                   colour = 'green4') + 
      geom_text_repel(size = 3,
                      aes(label = down.arrows$GENE),
                      color = 'black',
                      segment.color = 'black') +
      theme_classic(base_size = 16)
  </code></pre>
  
  <p>After that I would like to merge the points in 1 ggplot, so it's important to distinguish <code>up</code> and <code>down</code> </p>