1. A method for an interactive modification of a production plan for use with a computer system, the production plan comprised of a plurality of product tasks, each product task having two or more associated primitive tasks, with each primitive task having an associated primitive task information, comprising the steps of: specifying primitive task information for each primitive task, said primitive task information comprised of a time interval and a production quantity; storing said primitive task information for each of said plurality of product tasks on a storage device; displaying the production plan on a display device using a graphical representation for each primitive task, wherein the graphical representation for each primitive task is determined using the respective primitive task information and primitive task information of other primitive tasks that have overlapping time intervals, wherein each primitive task is graphically represented by at least one geometric object, and the primitive tasks of each product task are displayed having a common visual characteristic; establishing a plurality of modification rules for each product task, said rules defining how a change in the primitive task information of each primitive task associated with the product task, changes the primitive task information of other primitive tasks associated with the product task and said modification rules require that a total production quantity obtained by summing the production quantities for all primitive tasks associated with the first product task remain unchanged as a result of applying the modification rules; storing said plurality of modification rules on a storage device; modifying said production quantity of said primitive task information associated with a first primitive task where said modification is performed by manipulating the graphical representation of the first primitive task; modifying said production plan by changing the primitive task information of other primitive tasks based on the modification of the production quantity of said first primitive task and the modification rules of the product task with which the first primitive task is associated; and updating the display device to reflect modifications of the production plan.