title: ggplot in plotly missing legend
body: <p>I am having trouble getting a legend to show up in plotly, specifically with <code>ggplot + geom_line</code>.</p>
  
  <p>I am using <code>plotly 3.6.0</code> and <code>ggplot2 2.1.0</code></p>
  
  <p>To demonstrate my problem I'll build a simple data frame:</p>
  
  <pre><code>x = 1:5
  y1 = 1:5
  y2 = y1 * 2
  d = data.frame(x, y1, y2)
  
  g = ggplot(data = d, aes(x=x)) +
  geom_line(aes(y = y1, col = 'y1')) + 
  geom_line(aes(y = y2, col = 'y2')) + 
  scale_color_manual(values = c('red', 'blue'), labels = c('y1 lab', 'y2 lab'), name = '')
  g
  </code></pre>
  
  <p>This creates a ggplot figure with a legend. However, when I attempt to create an interactive version using plotly the legend is no longer there.</p>
  
  <pre><code>ggplotly(g)
  </code></pre>
  
  <p>I attempted the following to solve the problem:</p>
  
  <pre><code>g_build = plotly_build(g)
  g_build$layout$showlegend &lt;- TRUE
  g_build$layout$margin &lt;- list(l=80, r=300, b=80, t=100, pad=0)
  g_build
  </code></pre>