It's [evaluate][1]. That was the function I was looking for. It `Evaluates the given function in the context of the web page`. 

Finally, it's working:

    page.content = content; 
    page.injectJs('lib/d3/d3.min.js');
    page.evaluate(function() {
        var svg = d3.select('#graph')
                    .append('svg')
                    .attr({'width': 100, 'height': 100});   
    
        var circle = svg
            .append('circle')
            .attr({ 'cx': 10, 'cy': 10, 'r': 10, 'fill': 'blue' });   
    
        page.render('test.png');
        phantom.exit();
    };


  [1]: http://phantomjs.org/api/webpage/method/evaluate.html