ability to extend lines for thresholds and axes

In plotly, sometimes the lines don't go far enough, and it would be helpful to extend them.

Case 1: Multiple axes are not joined together. The x-axis line should connect all axes, not just the first 2. In the example image, I have drawn a red line where I think the x-axis should continue.

Case 2. I sometimes want to draw a target line on a chart; in this case, below the line would be "bad", resulting in a red bar, while above the line, would be good. To implement these, I have used a dedicated series, with a constant value. However, a target line should really a special case of a "grid" line, rather than a special case of a "data series", and it ought to extend right the way to the (appropriate) axis. I've drawn this in yellow on the chart below. (Even better, a target should be defined by a single value, rather than an array of repeated values).

Thanks very much :-)

Start by reproducing the two chart cases described in the issue: connecting an x-axis across multiple axes and extending a horizontal target or grid line to the appropriate axis. Review Plotly.js's axis, grid-line, and series rendering entry points to determine whether these require separate behaviors. Done means both cases work without representing a target as a repeated data series, with tests covering the resulting line extents.