Techniques for presenting data visualizations are described. Consistent with some embodiments of the present invention, a data visualization system includes an approximate query engine to derive approximate query results and present approximate data visualizations, in interactive time. The data visualization system also includes a precise query engine, thereby enabling a data analyst to selectively request that a precise query result be generated for a specific query, so that the data analyst can confirm a data observation made when viewing the approximate visualization for that specific query. The precise visualization is presented in a manner that allows the data analyst to view, and thus confirm or disprove, a data observation recorded when viewing the approximate visualization, and also compare the precise visualization with the previously generated and presented approximate visualization.