Y-axis Grid line on subchart

As 'regions' (along with zoom) are not working on the main chart, it would be nice to see the 'regions' (or even just grid line, indicating the region boundary) functionality on the sub-chart.

Please, look into it.

Start by tracing how regions and zoom are handled for the main chart, then compare that path with the sub-chart rendering. Determine how a region boundary or grid line could be represented on the sub-chart, and verify that the boundary is visible and aligned with the corresponding region.

Written by the indexing model from the issue text.