title: ggplot extend legend colorbar
body: <p>I'm trying to make a heatmap on ggplot. I have a data frame similar to the one below</p>
  
  <pre><code>wday    hour    quant
  Mon 0   1.2346944
  Tue 0   0.8418860
  Wed 0   0.8904375
  Thu 0   0.8906767
  Fri 0   1.0761553
  Sat 0   2.1095617
  Sun 0   2.1421696
  Mon 1   2.9178615
  Tue 1   0.7481875
  Wed 1   0.6200556
  Thu 1   0.5530000
  Fri 1   0.3852611
  Sat 1   0.4791192
  Sun 1   1.0958043
  Mon 2   2.8627222
  Tue 2   0.7989769
  Wed 2   0.4209105
  Thu 2   0.6512810
  Fri 2   0.5047176
  Sat 2   0.6544059
  Sun 2   0.8167846
  </code></pre>
  
  <p>And my code is :</p>
  
  <pre><code>ggplot(df , aes(x = hour, y = wday)) +
    geom_raster(aes(fill = quant), interpolate=TRUE) +
    scale_fill_distiller(palette = "Spectral") + 
    theme(legend.position = "bottom")
  </code></pre>
  
  <p>it renders a graph like that :</p>
  
  <p><a href="https://i.stack.imgur.com/w3oUW.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/w3oUW.png" alt="enter image description here"></a></p>
  
  <p>You can see that the colorbar of the legend is really contracted, how can I extend the legend horizontally so it becomes wider?</p>