title: Ternary plot and filled contour - ggtern
body: <p>I am trying to create a ternary contour filled plot with the library ggtern. The code used is</p>
  
  <pre><code>a &lt;- c(0.50, 0.625, 0.375, 0.25, 0.5625, 0.125, 0.25, 0.3125, 0.375, 0.4375, 0.1875, 0.3125, 0.375, 0.4375)
  b &lt;- c(0.25, 0.1875, 0.3125, 0.375, 0.25, 0.4375, 0.25, 0.375, 0.375, 0.4375, 0.875, 0.3125, 0.25, 0.125)
  c &lt;- c(0.25, 0.1875, 0.3125, 0.375, 0.1875, 0.4375, 0.50, 0.3125, 0.25, 0.125, 0.625, 0.375, 0.375, 0.4375) 
  d &lt;- c(77.82325, 74.59318767, 76.76495015, 76.62122282, 77.95608657, 76.91320817, 68.50986659,8.53724416,80.32237597, 85.43315399, 61.80292426, 74.71471485, 73.27176908, 67.51782848)
  df &lt;- data.frame(a, b, c, d)
  
  df$id &lt;- 1:nrow(df)
  
  #Build Plot
  ggtern(data = df,aes(x = c,y = a,z = b)) +
  stat_density2d(geom = "polygon", n = 400, aes(fill = ..level.., weight = d, alpha = abs(..level..))) +
  geom_density_tern(aes(weight = d,color = ..level..), n = 400) +
  geom_point(aes(fill = d),color = "black",size = 5,shape = 21) +
  geom_text(aes(label = id),size = 3) +
  labs(x = "X (%)",y = "Y (%)",z = "Z (%)",title = "Title", size = 3) +
  scale_fill_gradient2(low = "green", mid = "yellow", high = "red", midpoint = 10) +
  scale_color_gradient2(low = "green", mid = "yellow", high = "red", midpoint = 10) +
  theme_custom(base_size = 12, base_family = "", col.T = "black", col.L = "black", col.R = "black", col.BG = "white") +
  tern_anticlockwise() +
  tern_limits(breaks = seq(0.1,1,by = 0.1)) + #AFFECT ALL SCALES
  theme(axis.tern.arrowstart = 0.4,axis.tern.arrowfinish = 0.6) +
  theme(legend.justification = c(0,1), legend.position = c(0,1)) +
  guides(fill = guide_colorbar(order = 1), alpha = guide_legend(order = 2), color = "none") +
  labs( title = "Ternary filled contour plot", fill = "Value, V",alpha = "|V - 0|")
  </code></pre>
  
  <p>But I get the following warning message:</p>
  
  <pre><code>Error: coord_tern requires the following missing aesthetics: z
  </code></pre>
  
  <p>Why does this error occur and how to fix it? Please help</p>