Only those requests that are marked as Key Requests on services can be charted on dashboards right now. There is as well EAP for making custom metrics based on transactional data, but it is not GA available. In such case you will be able to create multidimensional analysis view which will be storing custom metrics made based on Web requests. Such metrics will be able to show up on dashboards. Top Web requests from diagnostic tools cannot be charted.
Here article about this feature, you can try to contact with the autor and ask about it. I don't have such informations so far.