Fix plot composition (#6206)

* Fixed composition

* Remove unnecessary guard code

* Removing deprecated code

* Use valid key for stacked plot v-for

* Fixed object API specs to expect old values as well as new values in mutation callbacks

* Fixed existing tests

* Added E2E test

* Fixed linting error

---------

Co-authored-by: Shefali Joshi <simplyrender@gmail.com>

src/api/objects/ObjectAPI.js

@@ -648,7 +648,7 @@ export default class ObjectAPI {
      * @param {module:openmct.DomainObject} object the object to observe
      * @param {string} path the property to observe
      * @param {Function} callback a callback to invoke when new values for
-     *        this property are observed
+     *        this property are observed.
      * @method observe
      * @memberof module:openmct.ObjectAPI#
      */