title: ggplot2 plotting two variables of two groups and with different scales
body: <p>I have a dataframe of following form:</p>
  
  <pre><code>School_type     Year    fund  rate  
      1           1998    8     0.1
      0           1998    7     0.2
      1           1999    9     0.11
      0           1999    8     0.22
      1           2000    10    0.12
      0           2000    15    0.23
  </code></pre>
  
  <p>I am thinking about plotting the "fund" and "rate" for each school type and the x axis is year, so there are four lines--two higher lines and two lower lines, but I don't know how to implement this with two scales of y-axes. Thanks in advance.</p>