Ability to extend extra lines through chart borders

I'd like to be able to extend an extra vertical line through the borders of the chart area until the end of the reserved space on the bottom. Might be useful in general to extend both vertical and horizontal line up until the end of the reserved size on each end of the chart.

No file, test, or entry point is named in the issue. Start by locating the chart-border rendering code in the Flutter chart implementation and identify how reserved space is calculated; done means vertical and horizontal lines can extend through the chart borders to the corresponding reserved-space edges.

Written by the indexing model from the issue text.