fallback to method = "lm" in absence of {mgcv}

Author: teunbrand

R/stat-smooth.R

@@ -111,7 +111,7 @@ StatSmooth <- ggproto("StatSmooth", Stat,
     }
 
     if (is.null(params$formula)) {
-      if (identical(method, "gam")) {
+      if (identical(method, "gam") && is_installed("mgcv")) {
         params$formula <- y ~ s(x, bs = "cs")
       } else {
         params$formula <- y ~ x
@@ -133,7 +133,15 @@ StatSmooth <- ggproto("StatSmooth", Stat,
     }
     # If gam and gam's method is not specified by the user then use REML
     if (identical(method, gam_method())) {
-      params$method.args$method <- params$method.args$method %||% "REML"
+      if (identical(method, stats::lm)) {
+        cli::cli_warn(c(
+          "The {.pkg mgcv} package must be installed to use \\
+          {.code method = \"gam\"}.",
+          "Falling back to {.code method = \"lm\"}."
+        ))
+      } else {
+        params$method.args$method <- params$method.args$method %||% "REML"
+      }
     }
 
     if (length(msg) > 0) {
@@ -204,4 +212,10 @@ StatSmooth <- ggproto("StatSmooth", Stat,
 )
 
 # This function exists to silence an undeclared import warning
-gam_method <- function() mgcv::gam
+gam_method <- function() {
+  if (is_installed("mgcv")) {
+    mgcv::gam
+  } else {
+    stats::lm
+  }
+}