gistfile1.js

var page = new WebPage(),
    address, selector, filename;

var system = require('system');
page.onConsoleMessage = function(msg) {
    system.stderr.writeLine('console: ' + msg);
};

//capture and captureSelector functions adapted from CasperJS - https://github.com/n1k0/casperjs
capture = function(targetFile, clips) {
    clips.forEach(function(clip, i) {
        if (typeof clip != "object") {
//            console.log(JSON.stringify(clip));
            throw new Error("clip must be an Object instance");
        }
        var targetFilename = targetFile + '-' + i + '.png';
//        console.log('Capturing page to ' + targetFile + ' with clipRect' + JSON.stringify(clip), "debug");
        page.clipRect = clip;
        try {
            page.render(targetFilename);
        } catch (e) {
            console.log('Failed to capture screenshot as ' + targetFilename + ': ' + e, "error");
        }
    });
    return this;
}

captureSelector = function(targetFile, selector) {
//    var selector = selector;
    var clips = page.evaluate(function(args) {
        try {
            var tables = document.querySelectorAll(args.selector);
            return [].map.call(tables, function(el) {
                var clipRect = el.getBoundingClientRect();
                return {
                    top: clipRect.top,
                    left: clipRect.left,
                    width: clipRect.width,
                    height: clipRect.height
                };
            });
        } catch (e) {
            console.log("Unable to fetch bounds for element " + selector, "warning");
        }
    }, { selector: selector });
    console.log(JSON.stringify(clips));
    return capture(targetFile, clips);
}

if (phantom.args.length < 2 || phantom.args.length > 5) {
    console.log('Usage: render_selector.js URL SELECTOR FILENAME_PREFIX');
    phantom.exit();
} else {
    address = phantom.args[0];
    selector = phantom.args[1];
    filename = phantom.args[2];

    page.onError = function (msg, trace) {
        console.log("error on chart address: " + address);
        console.log(msg);
        trace.forEach(function(item) {
            console.log('  ', item.file, ':', item.line);
        });
    }

    // add a little extra space for scroll bars and such
    page.viewportSize = { width: 1140, height: 1140 };
//    page.paperSize = { width: 1140, height: 1140, border: '0px' }

    page.open(address, function (status) {
        if (status !== 'success') {
            console.log('Unable to load the address!');
        } else {
            window.setTimeout(function () {
                captureSelector(filename, selector);
                phantom.exit();
            }, 200);
        }
    });
}