Allow mixture of milestone and timeframe in roadmaps

We have created roadmaps showing initiative timeframes as the cells, against Goals on the Rows, shown in Quarters.

It would be great to be able to add 'Releases' to this roadmap., but as milestone dots or vertical lines, rather than other time frames, as that makes no sense. 

This would make roadmaps far more usable and more matching traditional 'Smartie' or 'Swimlane' roadmaps.