The present invention provides a method implemented by a computer for generating an image on a display that represents a plurality of transactions as lines connecting points in the display indicative of a source and an origin for the respective transactions. For each of the lines, an anti-aliasing process is used to determine a coverage value for each pixel of the display that represents the extent to which the line covers each pixel, and a record of a cumulative coverage value for each pixel is maintained in an image buffer. When the lines have been rendered into the image buffer, the method comprises determining a maximum and a minimum of the cumulative coverage values stored in the image buffer, scaling the cumulative coverage value for each pixel, and rendering each pixel onto the display by using the scaled cumulative coverage value to determine a transparency for the pixel.