Adding a custom bar to visualise Y values at specific thresholds as colours

I’m trying to put together a chart that adds a bar at the bottom, to visualise Y values going over specific thresholds (in this example, below 20 is Green, 20 to 30 range is Yellow, and over 30 is Orange), as shown here:

The bar scales along with the line when zooming the chart

The version pictured builds the bar using box annotations, each defined as below:

new BoxAnnotationViewModel()
{
    X1 = currentXValue,
    X2 = currentXValue, + xValuesStepSize,
    Y1 = 0.96,
    Y2 = 1,
    Background = backgroundColor, //Colour calculated based on Y value
    AnnotationCanvas = AnnotationCanvas.AboveChart,
    CoordinateMode = AnnotationCoordinateMode.RelativeY
};

This implementation works, but the issue is that the bar is drawn inside the main chart area, and so covers some of the chart itself; if this could just be moved onto the X axis, rather than the chart canvas, then this would be exactly what I need.

I tried to just switch the canvas on the BoxAnnotations to use AnnotationCanvas.XAxis, but when doing this the annotations don’t seem to be getting the right X / Y coordinates for positioning – the width and height of each block looks correct, and the width scales when the chart is zoomed in, but every block appears to be positioned at 0,0 on the XAxis canvas (it looks like X1/Y1 values are ignored, so all blocks are stacked on top of each other in the same position rather than appearing alongside each other as a bar)

With a bit of searching I found an old post (https://www.scichart.com/questions/wpf/annotation-on-axis) stating that ‘AxisMarkerAnnotation’ is the only supported annotation for display on an axis canvas, so I tried using this instead, which does seem to respect the X1 position. Here though, the X2 value seems be be interpreted as just a static pixel width, and the width of the annotations does not scale when the chart is zoomed, so the blocks stay as whatever width is initially drawn and get moved further apart the more the chart is zoomed in.

I’m new to SciChart so hopefully I’m missing something obvious; is there any way of either getting the BoxAnnotation to work on an axis canvas, or of setting an AxisMarkerAnnotation’s width to be the width between two points on the X axis, scaling along with the chart when zooming?

I hope the above makes sense, thanks in advance for your help.