A different colored line chart above/below a threshold

By using the clip() function the Line chart can be drawn in two passes so that it's colored differently on each pass.

[No canvas support]

Update
There's information on this HOWTO page about creating dual color Line charts using gradients. It's very simple and can be done using either the RGraph Gradient() shortcut or more directly with the canvas linear gradient functionality.

This goes in the documents header:
<script src="RGraph.common.core.js"></script>
<script src="RGraph.line.js"></script>
Put this where you want the chart to show up:
<canvas id="cvs" width="600" height="250" !style="border: 1px solid #ccc">
    [No canvas support]
</canvas>
This is the code that generates the chart:
<script>
    var data      = [14,16,18,19,29,28,29,35,34,38,41,45,56,58,65];
    var threshold = 40;
    var color1    = 'red';
    var color2    = 'green';

    // First draw a chart that will enable us to get the Y coordinate.
    // This chart is not shown
    var line = new RGraph.Line({
        id: 'cvs',
        data: data,
        options: {
            marginTop: 5,
            marginBottom: 45,
            linewidth: 3,
            tickmarksStyle: 'endcircle',
            hmargin: 10,
            colors: [color1]
        }
    }).draw();

    // Now get the Y coordinate
    var y = line.getYCoord(threshold);

    // Clear the canvas
    RGraph.Clear(line.canvas);

    // Now draw the chart that will become the red half
    line.context.save();
        line.context.beginPath();
        line.context.rect(0,0,line.canvas.width, y);
        line.context.clip();

        // Draw the Line chart again
        line.draw();

    line.context.restore();

    // Now draw the chart that will become the green half
    line.context.save();
        line.context.beginPath();
        line.context.rect(0,y,line.canvas.width, line.canvas.height);
        line.context.clip();

        var line = new RGraph.Line({
            id: 'cvs',
            data: data,
            options: {
                marginTop: 5,
                marginBottom: 45,
                colors: [color2],
                linewidth: 3,
                tickmarksStyle: 'endcircle',
                hmargin: 10,
                textAccessible: true,
                xaxisLabels: ['Alf','Bert','Charlie','Dave','Edgar','Fliss','Gary','Hoolio','Indigo','Jack','Kevin','Lou','Martha','Neil','Olga']
            }
        }).draw();
        
    line.context.restore();
</script>