The MCSV format can be a useful API. When the MCSV input is a URL, the file can be static or a script that accesses a database to generate MCSV data.
The $entity_type and $entity_id place holders can be used to pass data to the MCSV URL. Their values will initially be replaced by the values in the Initial Entity Id and Initial Entity Type text fields.
When you double click on a node in the graph, more data will be loaded from the MCSV URL. The type and id of the double clicked node will be passed to the MCSV URL so that the retrieved data will be related to the double clicked node.
Google_Mcsv_Api.php is a sample file provided by TouchGraph to illustrate the kind of API that can be created. You can put this file on your server (the server needs to be capable of executing PHP). You can then specify the path to the php file as the Mcsv URL. The Google_Mcsv_Api.php receives parameters in the form of Google_Mcsv_Api.php?q=$entity_id&type=$entity_type. Type can either be "keyword" or "website". If type is "keyword", the api will return websites matching the "q" parameter. When type is "website", the api will return websites that are similar to and that have backlinks to the the website specified as the "q" parameter. The names of the entity types created with this API must be "Website" and "Keyword" because this is the value that will be set for entity_type when a node is double clicked.