The Tcl and Tk toolkit is in the public domain, and has a large user community. It provides familiar interface widgets such as buttons, labels and scrollbars which link to each other and processes. Each have characteristics such as colour, size, text and a command to run. It also contains the special canvas widget class. Canvases allow structured graphical items to be located and plotted.
Many graphical items are available including ovals, lines, polygons, and curves.
These relate closely to Bertin's visual variables which cartographers use to represent numerous attributes in a single symbol. Digital spatial information can be used to locate canvas items in order to draw maps. Dynamic maps can be created as an interface builder requires that widget properties can be changed instantly by other widgets. Thus a slider bar could be used to specify time and an circle item used to represent percentage unemployment. As the slider is moved, time changes, and employment figures read from an array can be used to re- configure the radius or colour of the symbol to produce a time series map.