feature: Adds support for variable interpolation

Author: NeedleInAJayStack

src/README.md

@@ -24,23 +24,24 @@ one by selecting `+ New Dashboard` from the Dashboard menu in the left panel.
 Once within the panel editor, select your Haystack data source in the Data Sources menu. Next, select the type of
 Haystack query that should be performed. The supported queries are:
 
-- Eval: Evaluate a free-form Axon expression. Grafana variables may be injected into the query, with the supported
-variables listed below. *Note: Not all Haystack servers support this functionality*
+- Eval: Evaluate a free-form Axon expression. *Note: Not all Haystack servers support this functionality*
 - HisRead: Display the history of a single point over the selected time range.
 - Read: Display the records matching a filter. Since this is not timeseries data, it can only be viewed in Grafana's
 "Table" view.
 
 #### Variables
 
-Some queries support injecting values from the Grafana UI. The following variables are supported:
+Grafana template variables can be injected into queries using the ordinary syntax, e.g. `$varName`.
+
+We also support injecting a few special variables from the time-range selector into the Eval and Read requests:
 
 - `$__timeRange_start`: DateTime start of the selected Grafana time range
 - `$__timeRange_end`: DateTime end of the selected Grafana time range
 - `$__maxDataPoints`: Number representing the pixel width of Grafana's display panel.
 - `$__interval`: Number representing Grafana's recommended data interval. This is the duration of the time range, 
 divided by the number of pixels, delivered in units of minutes.
 
-To use them, simply use the value in the input string. Below is an example of using the variables in an Eval query:
+To use them, simply enter the value in the input string. Below is an example of using the variables in an Eval query:
 
 ```
 > [{ts: $__timeRange_start, v0: 0}, {ts: $__timeRange_end, v0: 10}].toGrid

src/datasource.ts

@@ -1,5 +1,6 @@
-import { DataSourceInstanceSettings, CoreApp } from '@grafana/data';
-import { DataSourceWithBackend } from '@grafana/runtime';
+import { DataSourceInstanceSettings, CoreApp, ScopedVars, DataQueryRequest, DataQueryResponse } from '@grafana/data';
+import { DataSourceWithBackend, getTemplateSrv } from '@grafana/runtime';
+import { Observable } from 'rxjs';
 
 import { HaystackQuery, HaystackDataSourceOptions, DEFAULT_QUERY } from './types';
 
@@ -8,6 +9,15 @@ export class DataSource extends DataSourceWithBackend<HaystackQuery, HaystackDat
     super(instanceSettings);
   }
 
+  applyTemplateVariables(query: HaystackQuery, scopedVars: ScopedVars): Record<string, any> {
+    return {
+      ...query,
+      eval: getTemplateSrv().replace(query.eval, scopedVars),
+      hisRead: getTemplateSrv().replace(query.hisRead, scopedVars),
+      read: getTemplateSrv().replace(query.read, scopedVars),
+    };
+  }
+
   getDefaultQuery(_: CoreApp): Partial<HaystackQuery> {
     return DEFAULT_QUERY
   }