src/scale/Scale.js

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/**
 * // Scale class management
 * @module echarts/scale/Scale
 */
define(function (require) {

    var clazzUtil = require('../util/clazz');

    function Scale() {
        /**
         * Extent
         * @type {Array.<number>}
         * @protected
         */
        this._extent = [Infinity, -Infinity];

        /**
         * Step is calculated in adjustExtent
         * @type {Array.<number>}
         * @protected
         */
        this._interval = 0;

        this.init && this.init.apply(this, arguments);
    }

    var scaleProto = Scale.prototype;

    /**
     * Parse input val to valid inner number.
     * @param {*} val
     * @return {number}
     */
    scaleProto.parse = function (val) {
        // Notice: This would be a trap here, If the implementation
        // of this method depends on extent, and this method is used
        // before extent set (like in dataZoom), it would be wrong.
        // Nevertheless, parse does not depend on extent generally.
        return val;
    };

    scaleProto.contain = function (val) {
        var extent = this._extent;
        return val >= extent[0] && val <= extent[1];
    };

    /**
     * Normalize value to linear [0, 1], return 0.5 if extent span is 0
     * @param {number} val
     * @return {number}
     */
    scaleProto.normalize = function (val) {
        var extent = this._extent;
        if (extent[1] === extent[0]) {
            return 0.5;
        }
        return (val - extent[0]) / (extent[1] - extent[0]);
    };

    /**