title: Reversing default scale gradient ggplot2
body: <p>I am newbie, I am trying to desing a heat map. This is my code:</p>
  
  <pre><code>ggplot(gd, aes(Qcountry, Q6_1_Q6d), order = TRUE) +
    geom_tile(aes(fill = prob), colour = "white") +
    theme_minimal() +
    labs( y = "Main reason for mobility", x = "Country") +
    theme(axis.text.x = element_text(angle = 90, hjust = 1, vjust = 0.3)) +
    scale_fill_gradient(name = "(%)")
  </code></pre>
  
  <p>Which produces a perfect chart, my problem is low levels are dark blue, and higher values are light blue, which is not intuitive. Most common way to do is use <code>rev()</code>. But in my case I dont know how to. So, is it possible to reverse this <strong>default scale</strong>?
  <a href="https://i.stack.imgur.com/Wiy78.jpg" rel="noreferrer">This is the legend</a></p>
  
  <p>Other question, is there a way to create a scale gradient only with one colour. I mean, <code>scale_fill_gradient/scale_fill_gradientn</code>  need to set a low color and high color <code>(low = "", high = "")</code> and I want to change the blue by red.</p>
  
  <p>Thanks so much for your support.</p>