feat: add $schema property

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---

Author: kgryte

lib/node_modules/@stdlib/plot/vega/visualization/lib/defaults.js

@@ -32,6 +32,9 @@
 */
 function defaults() {
 	return {
+		// Schema URL:
+		'$schema': 'https://vega.github.io/schema/vega/v6.json',
+
 		// Coordinate axes:
 		'axes': [],
 

lib/node_modules/@stdlib/plot/vega/visualization/lib/main.js

@@ -27,6 +27,7 @@ var logger = require( 'debug' );
 var isPlainObject = require( '@stdlib/assert/is-plain-object' );
 var setReadOnly = require( '@stdlib/utils/define-nonenumerable-read-only-property' );
 var setReadWriteAccessor = require( '@stdlib/utils/define-read-write-accessor' );
+var setReadOnlyAccessor = require( '@stdlib/utils/define-read-only-accessor' );
 var hasProp = require( '@stdlib/assert/has-property' );
 var inherit = require( '@stdlib/utils/inherit' );
 var objectKeys = require( '@stdlib/utils/keys' );
@@ -69,6 +70,7 @@ var setProjections = require( './projections/set.js' );
 
 var getScales = require( './scales/get.js' );
 var setScales = require( './scales/set.js' );
+var getSchema = require( './schema/get.js' );
 var getSignals = require( './signals/get.js' );
 var setSignals = require( './signals/set.js' );
 
@@ -278,6 +280,21 @@ setReadOnly( Visualization.prototype, '_removeChangeListeners', function removeC
 	return this;
 });
 
+/**
+* Visualization schema URL.
+*
+* @name $schema
+* @memberof Visualization.prototype
+* @type {string}
+*
+* @example
+* var viz = new Visualization();
+*
+* var v = viz.$schema;
+* // returns '...'
+*/
+setReadOnlyAccessor( Visualization.prototype, '$schema', getSchema );
+
 /**
 * Visualization autosize configuration.
 *

lib/node_modules/@stdlib/plot/vega/visualization/lib/properties.json

@@ -1,4 +1,5 @@
 [
+  "$schema",
   "autosize",
   "axes",
   "background",

lib/node_modules/@stdlib/plot/vega/visualization/lib/schema/get.js

@@ -0,0 +1,38 @@
+/**
+* @license Apache-2.0
+*
+* Copyright (c) 2025 The Stdlib Authors.
+*
+* Licensed under the Apache License, Version 2.0 (the "License");
+* you may not use this file except in compliance with the License.
+* You may obtain a copy of the License at
+*
+*    http://www.apache.org/licenses/LICENSE-2.0
+*
+* Unless required by applicable law or agreed to in writing, software
+* distributed under the License is distributed on an "AS IS" BASIS,
+* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+* See the License for the specific language governing permissions and
+* limitations under the License.
+*/
+
+/* eslint-disable no-invalid-this */
+
+'use strict';
+
+// MAIN //
+
+/**
+* Returns the configuration schema URL.
+*
+* @private
+* @returns {string} schema URL
+*/
+function get() {
+	return this._$schema;
+}
+
+
+// EXPORTS //
+
+module.exports = get;

lib/node_modules/@stdlib/plot/vega/visualization/lib/to_json.js

@@ -44,9 +44,7 @@ function toJSON() {
 	var i;
 	var j;
 
-	out = {
-		'$schema': 'https://vega.github.io/schema/vega/v6.json'
-	};
+	out = {};
 
 	// Copy property values over to the output object...
 	for ( i = 0; i < PROPERTIES.length; i++ ) {