title: Setting factor level to be invisible in ggplot2
body: <p>I think there was an update to ggplot2 that I missed and some things have apparently changed.
  For a factor level of a variable to be omited or &quot;invisible&quot; you used to be able to set a named vector with the level set to NA in scale_color_manual as in the example below:</p>
  <pre><code>df_test &lt;- tibble(y = c(100, -50, 20),
                    group = c(&quot;A&quot;, &quot;B&quot;, &quot;C&quot;),
                    Marker = c(&quot;above average&quot;, &quot; &quot;, &quot;below average&quot;))
  
  ggplot(data = df_test, mapping = aes(x = group, y = y, color = Marker)) +
    geom_bar(stat = &quot;identity&quot;, position = position_dodge(width = 0.9), size = 1.5, fill = &quot;yellow&quot;) +
    scale_color_manual(values = c(&quot;above average&quot; = &quot;green3&quot;,
                                  &quot;below average&quot; = &quot;red&quot;,
                                  &quot; &quot; = NA))
  </code></pre>
  <p>Unfortunately, this doesnt work anymore and instead sets the factor level to gray instead.
  My desired output would be for both the &quot;B&quot; group to not have an outline and the factor level to not be shown in the legend.
  Using the &quot;alpha&quot; aesthetic does not work for me, as in my actual code I am allready using alpha for something else.</p>
  <p>Thanks in advance!</p>