Gantt chart

A Gantt chart gives every task a row of its own and draws its work as a bar lying along a shared date axis. Where the bar starts is when the task starts, how long it is is how long the task takes, and a line from the end of one bar to the start of another says the second cannot begin until the first is done. Read down the rows and you have the plan; read across a date and you have who is busy on it.

What makes it an interface rather than a picture is that all three of those facts are editable in place. A bar is dragged sideways to move the work, pulled by either end to change how long it takes (which is a resize handle doing ordinary work in an unusual place), and the rows themselves can be reordered or nested under a parent. The dependency lines then do the thing worth building the component for: move a bar and everything waiting on it moves too, so the plan stays a plan instead of quietly becoming a set of dates that contradict each other. A chart that lets a successor slide in front of its predecessor has drawn the arrows and thrown away their meaning.

Two smaller decisions decide whether it is readable. The axis needs a unit the plan is actually made in, days for a sprint and weeks or months for a year, with the weekend or the non-working days marked rather than silently included, since a five day bar that crosses a Saturday is not five days of work. And the number of rows on screen has a hard ceiling: past a couple of dozen the arrows cross each other more than they connect anything, which is the point where a real project tool starts collapsing groups.

Three neighbouring words get borrowed for this shape and none of them fit. The site’s timeline is a sequence of dated events, a history rather than a plan, and it has no notion of one entry waiting on another. A trace viewer draws bars along an axis too, but the axis is milliseconds and the bars are measurements of something that already ran. And a scheduler crosses days with hours so that an event lands in a slot, where this crosses tasks with dates so that a bar spans several: if the two views read alike, one of them has been built wrong. Confusingly, scheduler libraries often call the rows-of-resources view a “timeline”, and that view belongs here.