Agile Project Management has long done away with the Gantt Chart:
http://www.techdarkside.com/the-demise-of-the-gantt-chart-in-agile-software-projects
http://www.agilekiwi.com/earnedvalue/agile-charts/

Teambox is a very agile piece of software, so our team was excited about Teambox, but was disappointed when we saw the word "Gantt". Granted, Teambox does use a very light Gantt, which is wise, so it's arguable whether it should even be called a "Gantt". "Gantt" usually implies a large and complex, multi-tiered project plan with numerous dependencies and subdependencies - popular 15 years ago in the days of wasteful and slow Waterfall development.

A much more appropriate [valuable / modern] progress metric for Teambox would be Burn-Down Charts:
http://en.wikipedia.org/wiki/Burn_down_chart
Grab any Agile project management resource online to read more about them. Two critical pieces of data they reveal, that Gantt's don't, are Scope Creep and Effort Creep. Scope Creep is a powerful metric to have available. Stakeholders can see, in real-time, when they are causing delays to the timeline by adding features mid-iteration. Likewise, Effort Creep is valuable because you can immediately see when a User Story (Task) is beginning to exceed your estimates. (True, Gantt's can do this too, but it debatable whether they do it as well. IMO, they don't [15 years software-dev and project-management experience]).

If Teambox sticks with the Gantt-like chart, then they should consider simply removing the "Gantt" name - and perhaps call it something generic like "Progress Chart". "Gantt" is off-putting to many modern teams. I hope they aren't losing any sales over it. :-)