feat: add z-index support

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---

Author: kgryte

lib/node_modules/@stdlib/plot/vega/axis/ctor/lib/main.js

@@ -153,6 +153,9 @@ var setTicks = require( './ticks/set.js' );
 var getTitle = require( './title/get.js' );
 var setTitle = require( './title/set.js' );
 
+var getZIndex = require( './zindex/get.js' );
+var setZIndex = require( './zindex/set.js' );
+
 
 // VARIABLES //
 
@@ -1333,6 +1336,26 @@ setReadWriteAccessor( Axis.prototype, 'ticks', getTicks, setTicks );
 */
 setReadWriteAccessor( Axis.prototype, 'title', getTitle, setTitle );
 
+/**
+* Integer z-index indicating the layering of the title group relative to other axis, mark, and legend groups.
+*
+* @name zindex
+* @memberof Axis.prototype
+* @type {number}
+* @default 0
+*
+* @example
+* var axis = new Axis({
+*     'scale': 'xScale',
+*     'orient': 'bottom',
+*     'zindex': 2
+* });
+*
+* var v = axis.zindex;
+* // returns 2
+*/
+setReadWriteAccessor( Axis.prototype, 'zindex', getZIndex, setZIndex );
+
 /**
 * Serializes an instance to a JSON object.
 *

lib/node_modules/@stdlib/plot/vega/axis/ctor/lib/zindex/get.js

@@ -0,0 +1,43 @@
+/**
+* @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';
+
+// MODULES //
+
+var prop = require( './properties.js' );
+
+
+// MAIN //
+
+/**
+* Returns the integer z-index indicating the layering of the axis group relative to other axis, mark, and legend groups.
+*
+* @private
+* @returns {number} z-index
+*/
+function get() {
+	return this[ prop.private ];
+}
+
+
+// EXPORTS //
+
+module.exports = get;

lib/node_modules/@stdlib/plot/vega/axis/ctor/lib/zindex/properties.js

@@ -0,0 +1,33 @@
+/**
+* @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.
+*/
+
+'use strict';
+
+// MODULES //
+
+var property2object = require( '@stdlib/plot/vega/base/property2object' );
+
+
+// MAIN //
+
+var obj = property2object( 'zindex' );
+
+
+// EXPORTS //
+
+module.exports = obj;

lib/node_modules/@stdlib/plot/vega/axis/ctor/lib/zindex/set.js

@@ -0,0 +1,61 @@
+/**
+* @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';
+
+// MODULES //
+
+var logger = require( 'debug' );
+var isNumber = require( '@stdlib/assert/is-number' ).isPrimitive;
+var format = require( '@stdlib/string/format' );
+var changeEvent = require( './../change_event.js' );
+var prop = require( './properties.js' );
+
+
+// VARIABLES //
+
+var debug = logger( 'vega:axis:set:'+prop.name );
+
+
+// MAIN //
+
+/**
+* Sets the integer z-index indicating the layering of the axis group relative to other axis, mark, and legend groups.
+*
+* @private
+* @param {number} value - input value
+* @throws {TypeError} must be a number
+* @returns {void}
+*/
+function set( value ) {
+	if ( !isNumber( value ) ) {
+		throw new TypeError( format( 'invalid assignment. `%s` must be a number. Value: `%s`.', prop.name, value ) );
+	}
+	if ( value !== this[ prop.private ] ) {
+		debug( 'Current value: %s. New value: %s.', this[ prop.private ], value );
+		this[ prop.private ] = value;
+		this.emit( 'change', changeEvent( prop.name ) );
+	}
+}
+
+
+// EXPORTS //
+
+module.exports = set;