metrics: show legend with multiple datasets

src/metrics.js

@@ -355,7 +355,7 @@ async function readSystemFromGraphite(options) {
 }
 
 // CPU: stress --cpu 2 --timeout 60
-// Memory: stress --cpu 2 --timeout 60
+// Memory: stress --vm 2 --vm-bytes 256M
 async function getSystem(options) {
     assert.strictEqual(typeof options, 'object');