This page contains two Physlets, EField and DataGraph. The moving test charge in EField is a SDataSource that supplies the following variables to a SDataListener, the DataGraph in this case.
The DataGraph is a SDataListener. A DataGraph 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,"t","u");