Shapes layer: 'below' doesn't play well with subplots

[alexcjohnson]

I'm not sure this really has a solution even theoretically, if you consider the layering that needs to happen with inset plots (where the background of the inset plot needs to be drawn on top of the data of the main plot) but posting as a bug for discussion purposes since it's at the very least unexpected.

If you make a layer: 'below' shape that's data-referenced on one axis and paper-referenced on another, and it crosses two subplots (which could actually be a very nice way to highlight correlations between these subplots) the shape will NOT be below the data of both subplots.

Plotly.newPlot(gd,[
    {y:[1,2,3]},
    {y:[1,2,3],yaxis:'y2'}
],{
    yaxis:{domain:[0,0.4]},
    yaxis2:{domain:[0.6,1]},
    shapes:[{
        xref:'x',yref:'paper',type:'rect',
        x0:0.8,x1:1.2,y0:0,y1:1,
        fillcolor:'#eee',layer:'below'
    }]
})

[etpinard]

Nice find. I hope the shape_below_traces isn't wrong.

I'd vote for making layer: 'below' correspongd to below all traces linked to the axis set in the shape xref or yref.

[alexcjohnson]

closed by #1453 - now layer: 'below' shapes and images are still in front of plot backgrounds in all cases EXCEPT on inset plots (any subplot that overlaps an earlier one without being explicitly overlaying) where the component is NOT referenced to both axes of that subplot. So for example, if x2/y2 is on top of x/y, a shape referenced to x2/y2 can still be seen with layer: below but one referenced to x2/paper, paper/y2, x/y, or anything else cannot be seen where it intersects the x2/y2 subplot.

This is, I believe, the minimal set of objects it's theoretically impossible to show (short of some sort of horribly complicated stitching together of different objects in different z layers, which wouldn't work that well anyway due to antialiasing and would have strange effects of data objects from the lower plot slicing through them.)