When you use lighter render options there are some limitations due to how the lighter render mode works. When using bitmap, all series are drawn onto the same DrawingVisual. The drawing visual resides in a Canvas, lets call it a drawing-visual-canvas.
When you use the default xaml rendering, setting the ZIndex is directly applied to the series, so you can control its zindex in respect to all elements of the same level (all series, axes, annotations, track ball and the drawing-visual-canvas). So you can set the ZIndex of a xaml-rendered series and it will be drawn on top (or below) the drawing-visual-canvas regardless of the zindexes of the bitmap-rendered series. The ZIndex will be respected amongst the series with the same render mode. If I was unable to provide the information needed, please send to us your exact requirements and a snapshot of the desired result and I will try to guide you into getting this done.
The reason that the zero values of the line are not visible is not because the axis gets drawn on top of the series, but because the value is outside of the plot area. Due to that, the drawing visual resides completely between the axes and does not overlap them. This is partly a result of double calculations rounding, but as a final result I think that it is a flaw on our side and there should be a mechanism to allow for such a customization.I will put this under investigation to see if we can improve this behavior.
Currently you can not force the chart to extend the size of the drawing visual and you will not be able to avoid this clipping. The only work-around I can think of is to extend the range of the axis.
I hope this information helps, let me know if you have any other questions.
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