title: Removing "a" symbol from colour legend in ggplot2
body: <p>I keep getting this <code>a</code> on my colour legend when I make this graph in GGPLOT2.</p>
  
  <pre><code>ggplot(sher_ei_si, aes(SI, EI, shape = crop, label = treatment, colour =
          management)) +
          geom_point() +
          geom_text_repel(aes(SI, EI)) +
          xlim(0, 100) +
          ylim(0, 100) +
          labs(x = "Structure", y = "Enrichment", shape = "Crop", colour =
              "Management") +
          geom_vline(xintercept = 50) +
          geom_hline(yintercept = 50) +
          scale_colour_manual(values = c("grey0", "grey60")
  </code></pre>
  
  <p>Plot showing <code>a</code> under colour legend
  <a href="https://i.stack.imgur.com/kS5GZ.jpg" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/kS5GZ.jpg" alt="enter image description here"></a></p>