title: ggplot with variable line types and colors
body: <p>In R with ggplot, I want to create a spaghetti plot (2 quantitative variables) grouped by a third variable to specify line color.  Secondly, I want to aggregate that grouping variable with the line type or width.</p>
  
  <p>Here's an example using the airquality dataset. I want the line's color to represent the month, and the summer months to have a different line width from non-summer months.</p>
  
  <p>First, I created an indicator variable for the aggregated groups:</p>
  
  <pre><code>airquality$Summer &lt;- with(airquality, ifelse(Month &gt;= 6 &amp; Month &lt; 9, 1, 0))
  </code></pre>
  
  <p>I would like something like this, but with differing line widths:
  <a href="https://i.stack.imgur.com/hP6uL.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/hP6uL.png" alt="enter image description here"></a></p>
  
  <p>However, this fails:</p>
  
  <pre><code>library(ggplot2)
  ggplot(data = airquality, aes(x=Wind, y = Temp, color = as.factor(Month), group = Summer)) +
    geom_point() + 
    geom_line(linetype = as.factor(Summer))
  </code></pre>
  
  <p>This also fails (specifying <code>airquality$Summer</code>):</p>
  
  <pre><code>ggplot(data = airquality, aes(x=Wind, y = Temp, 
      color = as.factor(Month), group = airquality$Summer)) +
    geom_point() + 
    geom_line(linetype = as.factor(airquality$Summer))
  </code></pre>
  
  <p>I attempted <a href="https://stackoverflow.com/a/11714804/3935558">this solution</a>, but get another error:</p>
  
  <pre><code>lty &lt;- setNames(c(0, 1), levels(airquality$Summer))
  
  ggplot(data = airquality, aes(x=Wind, y = Temp, 
      color = as.factor(Month), group = airquality$Summer)) +
    geom_point() + 
    geom_line(linetype = as.factor(airquality$Summer)) + 
    scale_linetype_manual(values = lty)
  </code></pre>
  
  <p>Any ideas?</p>
  
  <p><strong>EDIT:</strong></p>
  
  <p>My actual data show very clear trends, and I want to differentiate the top line from all the others below.  My goal is to convince people they should make more than just the minimum payment on their student loans:
  <a href="https://i.stack.imgur.com/cQty9.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/cQty9.png" alt="enter image description here"></a></p>