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 :-)

首先复现 issue 中描述的两个图表案例：跨多个坐标轴连接 x 轴，以及将水平目标线或网格线延伸到适当的坐标轴。检查 Plotly.js 中坐标轴、网格线和数据系列的渲染入口点，以确定这些情况是否需要分别处理。完成的标准是两个案例都能正常工作，且无需将目标表示为重复的数据系列，并有测试覆盖最终线段的范围。