title: How to change the color key using Gadfly's `Scale.color_discrete_manual` in Julia?
body: <p>I have imported a <code>DataFrame</code> as below:</p>
  
  <pre><code>julia&gt; df
  100×3 DataFrames.DataFrame
  │ Row │ ex1     │ ex2     │ admit │
  ├─────┼─────────┼─────────┼───────┤
  │ 1   │ 34.6237 │ 78.0247 │ 0     │
  │ 2   │ 30.2867 │ 43.895  │ 0     │
  │ 3   │ 35.8474 │ 72.9022 │ 0     │
  │ 4   │ 60.1826 │ 86.3086 │ 1     │
  │ 5   │ 79.0327 │ 75.3444 │ 1     │
  │ 6   │ 45.0833 │ 56.3164 │ 0     │
  │ 7   │ 61.1067 │ 96.5114 │ 1     │
  │ 8   │ 75.0247 │ 46.554  │ 1     │
  ⋮
  │ 92  │ 90.4486 │ 87.5088 │ 1     │
  │ 93  │ 55.4822 │ 35.5707 │ 0     │
  │ 94  │ 74.4927 │ 84.8451 │ 1     │
  │ 95  │ 89.8458 │ 45.3583 │ 1     │
  │ 96  │ 83.4892 │ 48.3803 │ 1     │
  │ 97  │ 42.2617 │ 87.1039 │ 1     │
  │ 98  │ 99.315  │ 68.7754 │ 1     │
  │ 99  │ 55.34   │ 64.9319 │ 1     │
  │ 100 │ 74.7759 │ 89.5298 │ 1     │
  </code></pre>
  
  <p>I want to plot this <code>DataFrame</code> using <code>ex1</code> as x-axis, <code>ex2</code> as y-axis. In addition, the data is categorized by the third column <code>:admit</code>, so I want to give dots different colors based on the <code>:admit</code> value.</p>
  
  <p>I used <code>Scale.color_discrete_manual</code> to set up colors, and I tried to use <code>Guide.manual_color_key</code> to change the color key legend. However it turns out Gadfly made two color keys.</p>
  
  <pre><code>p = plot(df, x = :ex1, y = :ex2, color=:admit,
           Scale.color_discrete_manual(colorant"deep sky blue",
                                       colorant"light pink"),
           Guide.manual_color_key("Legend", ["Failure", "Success"],
                                  ["deep sky blue", "light pink"]))
  </code></pre>
  
  <p><a href="https://i.stack.imgur.com/6FOzI.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/6FOzI.png" alt="plot1"></a></p>
  
  <p>My question is how to change the color key legend when using <code>Scale.color_discrete_manual</code>?</p>
  
  <p>One related question is <a href="https://stackoverflow.com/questions/38615552/remove-automatically-generated-color-key-in-gadfly-plot">Remove automatically generated color key in Gadfly plot</a>, where the best answer suggests to use two layers plus <code>Guide.manual_color_key</code>. Is there a better solution for using <code>DataFrame</code> and <code>Scale.color_discrete_manual</code>?</p>