A computer implemented method for displaying data in the form of a heatmap comprises generating and displaying a first heatmap based on a first dataset of video surveillance data, receiving a user input selecting an area of the first heatmap, generating and displaying a second heatmap based on a second dataset of video surveillance data, wherein the second dataset is a subset of the first dataset which is limited only based on the area selected by the user, and wherein the step of generating and displaying the second heatmap comprises recalibrating a colour range based on the second dataset.