The Online Histogram Presenter (OHP) is the ATLAS tool to display histograms produced by the online monitoring system. In spite of the name, the Online Histogram Presenter is much more than just a histogram display. To cope with the large amount of data, the application has been designed to minimise the network traffic; sophisticated caching, hashing and filtering algorithms reduce memory and CPU usage. The system uses Qt and ROOT for histogram visualisation and manipulation. In addition, histogram visualisation can be extensively customised through configuration files. Finally, its very modular architecture features a lightweight plug-in system, allowing extensions to accommodate specific user needs. After an architectural overview of the application, the paper is going to present in detail the solutions adopted to increase the performance and a description of the plug-in system.
Dotti, Andrea;  Adragna, Paolo;  Vitillo, Roberto A, E-mail: firstname.lastname@example.org 
- CERN, CH-1211 Genve 23 Switzerland (Switzerland)
- Physics Department, Queen Mary, University of London Mile End Road London E1 4RP UK (United Kingdom)
- INFN Sezione di Pisa, Ed. C Largo Bruno Pontecorvo 3, 56127 Pisa (Italy)