You could try overlaying the line chart with a Gantt style bar chart (with minimal 'decoration' and background transparent). The problem with this type of overlay is that is not easy to make on-graph selections and the graphs can get out of sync when selections are made elsewhere in the document. If you can control these issues, the overlay works OK, otherwise an extension will be required.
Tried this one I tried also the solution where these two graphs were on top of each other - meaning not overlaying, but one is at the top part of screen and another is at the bottom (difficult to explain by the way )
But the problem is exactly the the thing that the scale is difficult to sync between two different objects.
The ideal solution would be to have one object to handle both "visualizations" - but maybe it is possible with extensions only. And I don't have skills to code those (yet)..