function Legend(g,
                mw = 25,
                mh = 25,
                mp = 10)
{
    // public methods
    this.draw = function(marks)
    {
        var marks = group.selectAll('.legend')
            .data(marks)
            .enter()
            .append('g')
            .attr('class', 'legend');

        marks.append('rect')
            .attr('x', 0)
            .attr('y', function(d, i) {
                return (markHeight + markPadding) * i;
            })
            .attr('width', markWidth)
            .attr('height', markHeight)
            .attr('fill', function(d, i) {
                return d.color;
            });

        marks.append('text')
            .attr('transform', function(d, i) { 
                var x = markWidth + markPadding;
                var y = (markHeight + markPadding) * i;
                y += 16;
                return 'translate(' + x + ',' + y + ')';
            })
            .attr('class', 'legend-text')
            .text(function(d) {
                return d.category;
            });
    }

    // private variables
    var that = this;
    var group = g;
    var markWidth = mw;
    var markHeight = mh;
    var markPadding = mp;
}