title: In jquery flot chart, highlight only one particular grid line
body: <p>I am using jquery flot chart to create a line chart. What I am not able to figure out is that how to change color of any one particular grid line.
  The whole thing - The chart shows the sensor readings. There are total of 4 different readings like - very low, low, high, very high. I want to highlight the grid lines corresponding to these four threshold values. 
  I am also providing a dummy image about what I really want to do --</p>
  
  <p><a href="http://i.imgur.com/uoiVhWt.png" rel="nofollow">This is the dummy I want to display</a></p>
  
  <p>The black line in this image is the grid color of that particular line(sorry for my bad drawing). Thanks in advance</p>