title: adding regression line per group with ggplot2
body: <p>I do the following graph:</p>
  
  <pre><code>&gt; ddd
        UV.NF TRIS            volvol
  2  145.1923   31  500 µl / 625  µl
  3  116.3462   50  500 µl / 625  µl
  4  127.1635   60  500 µl / 625  µl
  5  125.9615   69  500 µl / 625  µl
  6  162.0192   30    1 ml / 625  µl
  7  166.8269   50    1 ml / 625  µl
  8  176.4423   60    1 ml / 625  µl
  9  171.6346   70    1 ml / 625  µl
  19 292.3077   31 500 µl / 2500  µl
  20 321.1538   50 500 µl / 2500  µl
  21 225.0000   60 500 µl / 2500  µl
  22 263.4615   69 500 µl / 2500  µl
  23 301.9231   30   1 ml / 2500  µl
  24 350.0000   50   1 ml / 2500  µl
  25 282.6923   60   1 ml / 2500  µl
  26 282.6923   70   1 ml / 2500  µl
  35 133.6207   31  500 µl / 625  µl
  
  ggplot() +  
      geom_point(aes(y = log(UV.NF), x = TRIS, colour=ddd[,"volvol"], shape=ddd[,"volvol"]), 
          data=ddd) + 
      labs(colour = "volvol", shape="volvol") + xlab("TRIS (mM)") + 
      guides(colour = guide_legend(title="Vol. lyo. / Vol. reconst."), 
          shape=guide_legend(title="Vol. lyo. / Vol. reconst.")) +
      scale_shape_manual(values = c(19,19,3,3)) + scale_colour_manual(values = c(2,4,2,4))
  </code></pre>
  
  <p><img src="https://i.stack.imgur.com/eXPxE.png" alt="graph"></p>
  
  <p>I want to add the regression line <code>lm(y~x)</code> for each of the four groups appearing in the legend. I have done many attempts with <code>geom_smooth()</code> but without success.</p>