title: ggplot error Don't know how to automatically pick scale for object of type data.frame
body: <p>suppose the data frame is like this</p>
  
  <pre><code>df &lt;- data.frame(age = c("20", "21", "22", "20", "21","22"), 
     female = c("female", "female", "female", "male", "male", "male"), 
     income =c("1000", "2000","3000", "1500", "2400", "5600"))
  </code></pre>
  
  <p>I want to get two lines from ggplot that show me the comparison of income between female and male.
  I ran the following code:</p>
  
  <pre><code> p &lt;- ggplot(df, aes(x=age, y=income, group = female)) +
                           geom_line(color ="blue") 
  </code></pre>
  
  <p>I got the following error:
  Don't know how to automatically pick scale for object of type data.frame. Defaulting to continuous</p>
  
  <blockquote>
    <p>Error: Aesthetics must either be length one, or the same length as the
    dataProblems:female</p>
  </blockquote>
  
  <p>Anyone has idea what's wrong?</p>