Drag the square and notice the bar graph. The bar graph displays the electric flux passing through the cylinderical pipe.
This page contains two Physlets. The rectangle in EField is a SDataSource. It can supply the flux (as well as other variables) when it is dragged.
The Bar graph is a SDataListener. A Bar Physlet can display data supplied by a data source using a data connection. A data connection can be established between a data source and a listener using the following script:
document.EField.makeDataConnection(sourceID,listenerID,1,"f","f");
Only the first function string will be displayed in the Bar Physlet.