`selector` is a CSS selector or `'.'`. `attr` is either a `'fill'` or `'stroke'`. `gradient` must be in the special JointJS gradient format: `{ type: <linearGradient|radialGradient>, stops: [ { offset: <offset>, color: <color> }, ... ]`. An example is: `{ fill: { type: 'linearGradient', stops: [ { offset: '10%', color: 'green' }, { offset: '50%', color: 'blue' } ] } }`.
function(selector, attr, gradient) {

        var $selected = this.findBySelector(selector);

        // Generate a hash code from the stringified filter definition. This gives us
        // a unique filter ID for different definitions.
        var gradientId = gradient.type + this.paper.svg.id + joint.util.hashCode(JSON.stringify(gradient));

        // If the gradient already exists in the document,
        // we're done and we can just use it (reference it using `url()`).
        // If not, create one.
        if (!this.paper.svg.getElementById(gradientId)) {

            var gradientSVGString = [
                '<' + gradient.type + '>',
                _.map(gradient.stops, function(stop) {
                    return '<stop offset="' + stop.offset + '" stop-color="' + stop.color + '" stop-opacity="' + (_.isFinite(stop.opacity) ? stop.opacity : 1) + '" />'
                }).join(''),
                '</' + gradient.type + '>'
            ].join('');
            
            var gradientElement = V(gradientSVGString);
            if (gradient.attrs) { gradientElement.attr(gradient.attrs); }
            gradientElement.node.id = gradientId;
            V(this.paper.svg).defs().append(gradientElement);
        }

        $selected.each(function() {
            
            V(this).attr(attr, 'url(#' + gradientId + ')');
        });
    }