title: ggplot2: how to draw horizontal line on plot in log scale
body: <p>I have the folllowing dataframe:</p>
  
  <pre><code>dput(AR.df)
  structure(list(Type = structure(c(2L, 2L, 2L, 2L, 2L, 2L, 2L, 
  2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 1L, 1L, 1L, 
  1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 
  1L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 
  2L, 2L, 2L, 2L, 2L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 
  1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 
  2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 1L, 1L, 1L, 
  1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 
  1L), .Label = c("Complex-valued", "Magnitude-only"), class = "factor"), 
      AR.coef = c(1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 15, 20, 25, 30, 
      35, 40, 45, 50, 55, 60, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 15, 
      20, 25, 30, 35, 40, 45, 50, 55, 60, 1, 2, 3, 4, 5, 6, 7, 
      8, 9, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 1, 2, 3, 
      4, 5, 6, 7, 8, 9, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 
      60, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 15, 20, 25, 30, 35, 40, 
      45, 50, 55, 60, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 15, 20, 25, 
      30, 35, 40, 45, 50, 55, 60), variable = structure(c(1L, 1L, 
      1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 
      1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 
      1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 
      2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 
      2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 
      2L, 2L, 2L, 3L, 3L, 3L, 3L, 3L, 3L, 3L, 3L, 3L, 3L, 3L, 3L, 
      3L, 3L, 3L, 3L, 3L, 3L, 3L, 3L, 3L, 3L, 3L, 3L, 3L, 3L, 3L, 
      3L, 3L, 3L, 3L, 3L, 3L, 3L, 3L, 3L, 3L, 3L, 3L, 3L), .Label = c("p=0", 
      "p=1", "p=phat"), class = "factor"), value = c(0.07226, 0.08482, 
      0.08634, 0.0863, 0.08832, 0.08863, 0.08771, 0.08636, 0.08752, 
      0.08778, 0.08732, 0.08855, 0.08868, 0.08801, 0.08806, 0.08765, 
      0.08774, 0.08592, 0.0868, 0.08616, 0.08737, 0.09057, 0.08722, 
      0.08768, 0.08819, 0.08816, 0.08758, 0.08601, 0.08687, 0.08712, 
      0.08619, 0.08774, 0.08792, 0.08701, 0.08729, 0.08641, 0.08674, 
      0.08532, 0.08635, 0.08557, 0.05503, 0.05573, 0.05482, 0.0537, 
      0.05441, 0.05439, 0.05503, 0.05373, 0.05463, 0.0536, 0.05401, 
      0.05508, 0.05347, 0.05356, 0.05408, 0.05376, 0.05383, 0.05316, 
      0.0529, 0.05322, 0.05275, 0.05406, 0.05331, 0.05251, 0.05265, 
      0.0533, 0.05365, 0.0517, 0.05242, 0.05254, 0.05238, 0.0535, 
      0.05166, 0.05166, 0.05294, 0.0523, 0.05215, 0.05196, 0.05144, 
      0.05184, 0.06526, 0.06671, 0.06451, 0.06327, 0.06431, 0.06467, 
      0.06463, 0.06328, 0.0639, 0.06346, 0.06308, 0.06458, 0.06291, 
      0.063, 0.06351, 0.06288, 0.06372, 0.06227, 0.06239, 0.06268, 
      0.05505, 0.05666, 0.05595, 0.055, 0.05503, 0.05568, 0.05579, 
      0.05407, 0.05474, 0.05509, 0.05486, 0.05593, 0.05412, 0.0544, 
      0.05526, 0.05475, 0.05454, 0.0543, 0.05399, 0.05438)), row.names = c(NA, 
  -120L), .Names = c("Type", "AR.coef", "variable", "value"), class = "data.frame")
  </code></pre>
  
  <p>So, I try the following:</p>
  
  <pre><code>ggplot(AR.df, aes(x = AR.coef, y = value, linetype = variable, colour = Type)) +
      geom_line(size=1.25) + xlab("SNR") + ylab("False Positive Rate") +
          theme_bw() +
              theme(legend.justification=c(1,-0.2), legend.position=c(0.95,0.5), legend.text=element_text(size=11), legend.title=element_text(size=11), axis.title.x=element_text(size=14), axis.title.y=element_text(size = 14), legend.key = element_blank(), legend.background = element_rect(color="black",size = 0.1),legend.box = "horizontal") +
                  scale_linetype_manual(values=c(6,4,1), labels = expression(p==0, p==1, p==hat(p)),name="AR order")  +
                      scale_colour_manual(values=cbPalette, name="Analysis Type") +
                          theme(legend.key.width=unit(3,"line")) + ylim(c(0.04,0.1))  +
                              guides(fill=guide_legend(ncol=2))  + coord_trans(x="log2") + scale_x_continuous(breaks=c(1:10,10*(2:6))) + geom_abline(aes(intercept=0.05, slope = 0), linetype="dotted")
  </code></pre>
  
  <p>and I get the plot, but no horizontal line at 0.05, simply because <code>NaNs</code> are produced/Warning messages:</p>
  
  <blockquote>
    <p>1: In coord$trans$x$transform(x) : NaNs produced<br>
    2: In trans$transform(value) : NaNs produced</p>
  </blockquote>
  
  <p>Where do these <code>NaNs</code> come from, since my x-scale is from 1 to 60, so zero is not possible for log to evaluate (because it is not in the range).</p>
  
  <p>So, what am I doing wrong?</p>