title: lattice xyplot panel.abline - how to add different horizontal lines based on x values?
body: <p>How can I draw horizontal lines in each graph based on specified x values? For example, when I have X=1 on x-axis, the matched dot on the plot is (1,y0), and then consequently draw a horizontal line Y=y0? </p>
  
  <p>A working example using iris data with only a vertical line at x=0.5: </p>
  
  <pre><code>iris = read.table("http://archive.ics.uci.edu/ml/machine-learning-databases/iris/iris.data", sep=',', header=F)
  names(iris) = c("sepal.length", "sepal.width", "petal.length", "petal.width", "iris.type")
  
  
  xyplot(sepal.length ~ petal.width | iris.type, data = iris,
      panel = function(...) {
          panel.abline(v=0.5, lty = "dotted", col = "black")
          panel.xyplot(...)
  })
  </code></pre>
  
  <p>But I'd also want to have horizontal lines shown in this way:
  See in the <code>iris-setosa</code> graph(panel), a horizontal line is marked through the point at (0.5,y)--I draw this manually.   I don't know how to specify y in the panel.abline, since y is a variable that appears different for each panel. </p>
  
  <p>In my actual data, my x and y have one-to-one relation. </p>
  
  <p>I thought it should be a simple problem, but have no idea how to work this around. I hope this is clearer. </p>
  
  <p><img src="https://i.stack.imgur.com/SL4dh.png" alt="enter image description here"></p>