update to plotly.js 1.51.2 (#1988)

* update to plotly.js 1.51.2
* Fix doctest

Author: jonmmease

Diff for: packages/javascript/jupyterlab-plotly/package-lock.json

@@ -31,7 +31,7 @@
     "typescript": "~3.1.1"
   },
   "dependencies": {
-    "plotly.js": "^1.51.1",
+    "plotly.js": "^1.51.2",
     "@types/plotly.js": "^1.44.9",
     "@jupyterlab/rendermime-interfaces": "^1.3.0",
     "@phosphor/messaging": "^1.2.3",