1. A method in a computing system, the method comprising: displaying a visual representation of a prototype GUI; generating the prototype GUI by, receiving first information about a visual appearance of a first prototype GUI element; creating a first proxy GUI object to represent the first prototype GUI element, the created first proxy GUI object comprising the first visual appearance information; receiving second information about a visual appearance of a second prototype GUI element; creating a second proxy GUI object to represent the second prototype GUI element, the created second proxy GUI object comprising the second visual appearance information; updating the displayed visual representation of the prototype GUI to include; a visual representation of the first prototype GUI element that is based on the first visual appearance information from the created first proxy GUI object and that includes one or more placeholders for data from a first of a plurality of data sources; a visual representation of the second prototype GUI element that is based on the second visual appearance information from the created second proxy GUI object and that includes one or more placeholders for data from a second of the plurality of data sources, wherein the first and second data sources are distinct from each other; generating first and second actual GUI objects that correspond to the first and second proxy GUI objects, respectively, wherein the first and second actual GUI objects comprise the first and second visual appearance information from the first and second proxy GUI objects, respectively; modifying the first and second actual GUI objects to include first and second specifications, respectively, of the first and second data sources, respectively; and displaying an actual GUI in such a manner as to include first and second actual GUI elements corresponding to the first and second actual GUI objects, respectively, wherein the first and second actual GUI elements are displayed based on the first and second visual appearance information, respectively, from the first and second actual GUI objects, respectively, and wherein the first and second actual GUI elements comprise first and second visual representations, respectively, of data from the first and second data sources, respectively, so that the displayed actual GUI has a visual appearance similar to the updated displayed visual representation of the prototype GUI in addition to visual representation of data from the first and second data sources.