title: geom_contour mark specific contour (isobar) value
body: <p>I am attempting to plot sea level pressure (isobars) contour on an spatial area with 2mb spacing and want a specific isobar (1015) plotted as thicker line than the others and wonder if it is possible with other function and/or with ggplot/geom_conour related function.
  Below the command I sued to plot the isobar. Now any idea on how to add a oommand for the 1015 isobar?</p>
  <pre><code>syn_plot &lt;- ggplot() +
  geom_tile(data = synclas_gather_df, aes(x=x, y=y, fill=value)) +
  geom_sf(data = map_bg, fill=&quot;transparent&quot;)+
  geom_contour2(data = synclas_gather_df, aes(x=x,y=y,z=value), binwidth = 2, color = &quot;black&quot;) +
  scale_fill_gradientn(colours = colorRamps::matlab.like2(100), name = &quot;hPa&quot;,breaks=0:5) +
  scale_colour_gradient(guide = 'none') + facet_wrap(~key, ncol = 4) +
  scale_x_continuous(limits = c(-34,29), expand = c(0, 0))+
  scale_y_continuous(limits = c(-9,34), expand = c(0,0))+
   theme_bw() + theme(axis.title = element_blank(),
   axis.text = element_blank(),
   axis.ticks = element_blank())
  
    syn_plot + geom_text_contour(data= synclas_gather_df,aes(x,y,z = value), stroke = 0.10,binwidth = 4,size=3)
  </code></pre>
  <p>data format:</p>
  <pre><code> time                  lon   lat   slp
  </code></pre>
  <p>                
  1 1978-12-30 12:00:00   0      40 1015.
  2 1978-12-30 12:00:00   2.5    40 1013.
  3 1978-12-30 12:00:00   5      40 1012.
  4 1978-12-30 12:00:00   7.5    40 1010.
  5 1978-12-30 12:00:00  10      40 1007.
  6 1978-12-30 12:00:00  12.5    40 1005.
  7 1978-12-30 12:00:00  15      40 1004.
  8 1978-12-30 12:00:00  17.5    40 1003.
  9 1978-12-30 12:00:00  20      40 1002.
  10 1978-12-30 12:00:00  22.5    40 1001.</p>