make legend detection code more robust. fixes tidyverse#3963

Author: clauswilke

R/layer-sf.R

@@ -48,14 +48,16 @@ LayerSf <- ggproto("LayerSf", Layer,
 
     # automatically determine the legend type
     if (is.null(self$legend_key_type)) {
+      # first, set default value in case downstream tests fail
+      self$geom_params$legend <- "polygon"
+
+      # now check if the type is not polygon
       if (is_sf(data)) {
         sf_type <- detect_sf_type(data)
         if (sf_type == "point") {
           self$geom_params$legend <- "point"
         } else if (sf_type == "line") {
           self$geom_params$legend <- "line"
-        } else {
-          self$geom_params$legend <- "polygon"
         }
       }
     } else {