Modern tools for visualization of information can be very helpful for data analysis and discovery of patterns in data. The paper describes a framework for documenting of findings using freehand drawings on top of the displays representing data. These drawings or marks become a basis for the annotations of the analyzed information. The marks are used to designate patterns in data and to refer to data elements with interesting characteristics. A model of data visualization is described that allows encoding of marks in terms of the presented information. This brings independence of marks from the specifics of visualizing methods — the marks are adapted according to changes of the way the data is presented. Such representation of marks also gives the possibility for the system to provide the user with information and statistics about the data referred by marks and to initiate visualizations showing the referred data in other perspectives.