how to overlay a line on a dojo column chart

I am using a dojo column chart. I want to add a custom line (some kind of a threshold line) that is drawn on the plot. So, lets say y axis ranges from 0 to 5. I want a horizontal line at, lets say, 4.2 running across the plot. It is a column chart. I was hoping to find some drawing APIs that can help me do custom drawing on the plot but i am unable to figure out how. I know the chart uses gfx and surface so if i can get a handle to the chart/plot surface perhaps i can draw a custom line? Will also need data to rendered co-ordinates mapping to make this happen

My current chart uses code like:

var mychart = new dojox.charting.Chart2D("columns").
        addAxis("x", {fixLower: "minor", fixUpper: "minor", natural: true,
                            font: "normal normal 10pt Arial", 
                             labels: [{value: 1, text: "Q2 FY11"},
                                      {value: 2, text: "Q3 FY11"},
                                    {value: 3, text: "Q4 FY11"},
                                    {value: 4, text: "Q1 FY12"}]
                            }).
        addAxis("y", {vertical: true, includeZero: false, fixLower: "major", fixUpper: "major", min: 0, max: 5, font: "normal normal 10pt Arial", majorTick: {color: "black", length: 6}, minorTicks: false}).
        addPlot("default", {type: "ClusteredColumns", tension: "S", shadows: {dx: 2, dy: 2}, gap: 5, minBarSize : 14, maxBarSize:24, animate:  { duration: 1000, easing: dojo.fx.easing.linear} }).
        addSeries("Series A", [{ y: 2.3, tooltip: "FFFF"}, { y: 3.5, tooltip: "GGGG"}]).
        addSeries("Series B", [1.2, 2.5]);

Solution

You could render the line using another plot.

new dojox.charting.Chart2D("columns").
    addAxis("x", {fixLower: "minor", fixUpper: "minor", natural: true,
                        font: "normal normal 10pt Arial", 
                         labels: [{value: 1, text: "Q2 FY11"},
                                  {value: 2, text: "Q3 FY11"},
                                {value: 3, text: "Q4 FY11"},
                                {value: 4, text: "Q1 FY12"}]
                        }).
    addAxis("y", {vertical: true, includeZero: false, fixLower: "major", fixUpper: "major", min: 0, max: 5, font: "normal normal 10pt Arial", majorTick: {color: "black", length: 6}, minorTicks: false}).
    addPlot("default", {type: "ClusteredColumns", tension: "S", shadows: {dx: 2, dy: 2}, gap: 5, minBarSize : 14, maxBarSize:24, animate:  { duration: 1000, easing: dojo.fx.easing.linear} }).
    addSeries("Series A", [{ y: 2.3, tooltip: "FFFF"}, { y: 3.5, tooltip: "GGGG"}]).
    addSeries("Series B", [1.2, 2.5]).
    addPlot("threshold", { type: "Lines" }).
    addSeries("threshold", [{x: 0, y: 4.2}, {x: 4, y: 4.2}], { plot: "threshold" }).
    render();