Show Release Date (external) on Gantt chart

Currently , only the internal release date can feature in a Gantt chart for a given release. Our internal and external release dates are different, so we would like to indicate this on a Gantt chart - only showing the internal release date in symbol format does not show the full picture. The External Release date should have a different shaped symbol.

For people who have the same internal and external release date, the chart is ideal, but for me, it is missing some vital information before I can use this the way I want to.

Also having the ability to add a custom field, like is possible on a Custom roadmap, will enable additional important information to be displayed.

Thanks!