[Bug fix] Resolving key conflicts in Timeseries Annotation Layer when…

… key is a string (apache#4768)

* Fix how the annotation layer interpretes the timestamp string without timezone info; use it as UTC

* [Bug fix] Fixed/Refactored annotation layer code so that non-timeseries annotations are applied based on the updated chart object after adding all data

* [Bug fix] Fixed/Refactored annotation layer code so that non-timeseries annotations are applied based on the updated chart object after adding all data

* Fixed indentation

* Fix the key string value in case series.key is a string

* Fix the key string value in case series.key is a string

superset/assets/visualizations/nvd3_vis.js

@@ -543,7 +543,7 @@ function nvd3Vis(slice, payload) {
             return {};
           }
           const key = Array.isArray(series.key) ?
-            `${a.name}, ${series.key.join(', ')}` : a.name;
+            `${a.name}, ${series.key.join(', ')}` : `${a.name}, ${series.key}`;
           return {
             ...series,
             key,