Add solid as equivalent of linetype = 1.

Fixes #1534

Author: hadley

NEWS.md

@@ -1,5 +1,7 @@
 # ggplot2 2.0.0.9000
 
+* `geom_path()` knows that "solid" (not just 1) represents a solid line (#1534).
+
 * `layer()` now automatically adds a `na.rm` parameter if none is explicitly
   supplied.
 

R/geom-path.r

@@ -168,8 +168,9 @@ GeomPath <- ggproto("GeomPath", Geom,
 
     # Work out whether we should use lines or segments
     attr <- plyr::ddply(munched, "group", function(df) {
+      linetype <- unique(df$linetype)
       data.frame(
-        solid = identical(unique(df$linetype), 1),
+        solid = identical(linetype, 1) || identical(linetype, "solid"),
         constant = nrow(unique(df[, c("alpha", "colour","size", "linetype")])) == 1
       )
     })