title: Add fitted exponential equation into ggplot 2
body: <p>I'm having trouble showing the fitted exponential equation on a plot. I used ggplot2 and <code>stat_smooth()</code> to draw the points and fitted lines. Now I want to show the fitted equation on the plot but <code>stat_poly_eq()</code> does not work. Does anyone know how to do that? My code is attached here.</p>
  <pre><code>mydata = data.frame(&quot;conc_ensemble&quot; = c( 2.000000, 2.477121, 2.778151, 2.954243, 3.000000, 3.477121, 3.778151, 3.954243, 4.000000,
                                   4.477121, 4.778151, 4.954243, 5.000000, 5.477121, 5.778151, 5.954243, 6.000000), 
              &quot;REV&quot; = c(5.20000, 3.76000, 2.23000, 1.96000, 1.57500, 1.00000, 0.71250, 0.65000, 0.49000, 0.28300, 0.22900, 0.17500,
                        0.16225, 0.08830, 0.06075, 0.06075, 0.05350))
  
  my.formula = y ~ a*exp(b*x)
  ggplot(mydata, aes(conc_ensemble, REV) ) +
    geom_point() +
    stat_smooth(method = 'nls', method.args = list(start = c(a=1, b=1)), 
                formula = my.formula, colour = 'red', se = FALSE) +
    stat_poly_eq(formula = my.formula,
                 label.x.npc = 0.2,  size = 3,
                 aes(label = paste(..eq.label.., ..rr.label.., sep = &quot;~~~&quot;)), 
                 parse = TRUE,
                 rr.digits = 3)
  </code></pre>