revert test changes

Author: mbostock

test/plots/empty.js

@@ -6,11 +6,18 @@ export default async function() {
     grid: true,
     inset: 6,
     x: {
-      domain: "ABCDEFGH"
+      domain: [0, 1]
     },
     y: {
-      line: true,
       domain: [0, 1]
-    }
+    },
+    marks: [
+      Plot.frame(),
+      undefined,
+      null,
+      () => null,
+      () => undefined,
+      () => svg`<circle cx=50% cy=50% r=5 fill=green>`
+    ]
   });
 }