title: How to add multiple straight lines in a multi plot.zoo
body: <p>I have multiple time series data plots and I need an horizontal line in each plot but with different horizontal values (es. 1st plot: <code>h=50</code>, 2nd plot: <code>h=48</code>...).</p>
  
  <p>I tried <code>abline(h=50...</code> and I get the horizontal line in each plot.
  I tried <code>abline(h=c(50,48...</code> and I get multilple horizontal lines in each plot.</p>
  
  <p>I can't figure out how to get the plot.zoo index in order to plot <code>h=50</code> in the 1st plot, <code>h=48</code> in the 2nd plot and so on.</p>
  
  <pre><code>library(xts)
  data(sample_matrix)
  x &lt;- as.xts(sample_matrix)
  
  # plot with single line
  my.panel &lt;- function(x, ...) {
      lines(x, ...)
      abline(h=50, col = "red", lty="solid", lwd=1.5 )
  }
  plot.zoo(x, main="title",
  plot.type="multiple", type="o", lwd=1.5, col="blue",
  panel=my.panel)
  
  
  # plot multiple lines in all plots
  my.panel &lt;- function(x, ...) {
      lines(x, ...)
      abline(h=c(50,50,48,50), col = "red", lty="solid", lwd=1.5 )}
  
  plot.zoo(x, main="title",
  plot.type="multiple", type="o", lwd=1.5, col="blue",
  panel=my.panel)
  </code></pre>