title: Setting edge/border thickness using geom_sf in ggplot
body: <p>I am trying to change the border colour on regions of and sf object when I plot it. Changing the colour of the border is fine, however, the lines are a bit thin so I want to make only the coloured lines thicker so they are more visible.</p>
  
  <p>Reading <a href="https://stackoverflow.com/questions/47720329/how-can-i-remove-border-lines-when-using-geom-sf">this question</a> suggests that using "lwd" will allow changes in thickness. However, when I try I either get no effect or extremly thick lines.</p>
  
  <p>See the example below</p>
  
  <pre><code>library(sf)
  library(ggplot)
  library(dplyr)
  
  nc &lt;- st_read(system.file("shape/nc.shp", package="sf")) %&gt;%
    mutate(type= case_when(
      BIR74&gt;16000 ~"High",
      TRUE ~"Low"
    ) %&gt;% factor(levels = c("Low", "High"))) #the levels are ordered to avoid the grey lines overwriting the red ones
  
  nc %&gt;%
    ggplot(.) + 
    geom_sf(aes(fill = BIR74, colour = type)) + #does the job but the coloured borders are quite thing
    scale_color_manual(values = c( "#666666","#F8766D"))
  
  
  nc %&gt;%
    ggplot(.) + 
    geom_sf(aes(fill = BIR74, colour = type, 
                lwd = ifelse(type =="High", 1, 0.5)) #The values can be anything and it still looks rubbish
            ) + 
    scale_color_manual(values = c( "#666666","#F8766D"))
  </code></pre>
  
  <p><a href="https://i.stack.imgur.com/aGmli.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/aGmli.png" alt="enter image description here"></a></p>
  
  <p>How can I get only senible thicknesses like x% thicker? Ideally only the target edges will be changed.</p>