title: Changing the line types with trellis doubleYScale plot
body: <p>I did create a <code>doubleYScale</code> plot: </p>
  
  <pre><code>obj1 &lt;- xyplot(Produktion ~ Year, datac1mw, type = "l",col="black")
  obj2 &lt;- xyplot(Summa.skulder ~ Year, datac1mw, type = "p",col="black")
  
  hm=doubleYScale(obj1, obj2, add.ylab2 = TRUE,text = c("Produktion", "Summa.skulder"), 
               par.settings = simpleTheme(col = c('blue','black'), lty = 2:3))
  </code></pre>
  
  <p>The result I get is: 
  <img src="https://i.stack.imgur.com/WrO1x.jpg" alt="enter image description here"></p>
  
  <p>What I really want is a plot where the <code>legend</code> shows not 2 lines (as now a blue and a red line) but that Produktion (left y-axis) is dotted and Summa.Skulder (right y-axis) is a line, in the legend. </p>
  
  <p>Any suggestions?
  Best Regards</p>