title: How to overlay a line for an lm object on a ggplot2 scatterplot
body: <p>I have some data, </p>
  
  <pre><code>calvarbyruno.1&lt;-structure(list(Nominal = c(1, 3, 6, 10, 30, 50, 150, 250), Run = structure(c(1L, 
  1L, 1L, 1L, 1L, 1L, 1L, 1L), .Label = c("1", "2", "3"), class = "factor"), 
      PAR = c(1.25000000000000e-05, 0.000960333333333333, 0.00205833333333334, 
      0.00423333333333333, 0.0322333333333334, 0.614433333333334, 
      1.24333333333333, 1.86333333333333), PredLin = c(-0.0119152187070942, 
      0.00375925114245899, 0.0272709559167888, 0.0586198956158952, 
      0.215364594111427, 0.372109292606959, 1.15583278508462, 1.93955627756228
      ), PredQuad = c(-0.0615895732702735, -0.0501563307416599, 
      -0.0330831368244257, -0.0104619953693943, 0.100190275883806, 
      0.20675348710041, 0.6782336426345, 1.04748729725370)), .Names = c("Nominal", 
  "Run", "PAR", "PredLin", "PredQuad"), row.names = c(NA, 8L), class = "data.frame")
  calweight &lt;- -2
  </code></pre>
  
  <p>for which I've created both a linear and a quadratic lm model</p>
  
  <pre><code>callin.1&lt;-lm(PAR~Nominal,data=calvarbyruno.1,weight=Nominal^calweight)
  calquad.1&lt;-lm(PAR~Nominal+I(Nominal^2),data=calvarbyruno.1,weight=Nominal^calweight)
  </code></pre>
  
  <p>I can then plot my data values using ggplot2</p>
  
  <pre><code>qplot(PAR,Nominal,data=calvarbyruno.1)
  </code></pre>
  
  <p>But can't work out how to overlay a line representing the two lm objects... Any ideas ?</p>