Different methods to analyse variations in vessel size in tree rings with and without intra-annual density fluctuations (IADFs) in Erica arborea L. are presented. These methods are based on the in continuum collection of vessel size data within the ring using digital images. In the analysis of rings from the early- (EW) to late-wood (LW), the following vessel parameters were determined: (a) progressive number, (b) lumen area, and (c) lumen centre of gravity (i.e. distance between lumen centre and EW beginning). To make rings of different width or number of vessels comparable, progressive number and centre of gravity variables were standardised. Different graphical representations and data interpolation techniques were compared. Our results indicate that the most consistent procedure to measure the position and width of IADFs along tree rings should include the following steps: (a) plotting vessel lumen area in relation to standardised progressive number, (b) interpolation of vessel area series using simple moving average, (c) superimposition of curves from series of tree rings with and without IADFs, and (d) the establishment of the points where the two series intercept. Our results show that, in diffuse-porous woods, vessel position can be represented by a simpler automatically detected parameter, thus simplifying the procedure for data collection and analysis. The proposed graphical representation also facilitates the establishment of links between IADFs and ecological processes.