Add role for gnuplot.

nix/configuration/roles/gnuplot/default.nix

@@ -0,0 +1,29 @@
+{
+  config,
+  lib,
+  pkgs,
+  ...
+}:
+
+{
+  imports = [ ];
+
+  options.me = {
+    gnuplot.enable = lib.mkOption {
+      type = lib.types.bool;
+      default = false;
+      example = true;
+      description = "Whether we want to install gnuplot.";
+    };
+  };
+
+  config = lib.mkIf config.me.gnuplot.enable (
+    lib.mkMerge [
+      {
+        environment.systemPackages = with pkgs; [
+          gnuplot
+        ];
+      }
+    ]
+  );
+}