With increasing amounts of data and its significance in all areas, there is also a growing need to effectively present data – we can attempt that with data visualization. With this experiment we tried to check how understanding of shown data is influenced by different data presentations and their graphical elements. We tested effectiveness of a presentation in a table against different data visualizations. Additionally we also test the effect of following guidelines for use of colour in visualization. We created the list of guidelines based on existing literature for the purposes of this experiment. 37 students of the Natural Sciences and Engineering took part in the experiment. They were shown three datasets, one of which was in a table and two were data visualizations. One visualization followed the colour use guidelines, the other did not. The subjects were divided into three groups, so that each dataset could be shown in all three different ways. While viewing the data presentations they answered questions relating to the data, during which their gaze direction was being tracked. Data presentation effectiveness was being measured using number of wrong answers, speed of answering (measured from time of answering previous question or time of showing the presentation), and number and duration of fixations. We found that the influence on effectiveness depends on specific case and that we need to tailor the visualization accordingly, but on average all metrics showed a positive influence of visualizing data and usage of colour guidelines.