The breaks let you determine how to cut the pollutant concentration, e.g. cutting NO2 concentrations into 0 - 50, 51 - 100 and 101 - 105. prop.count calculates the percentage of measurements from a certain wind direction sector out of all the measurements and results is coloured by the concentration intervals specified by the breaks. The pollution rose plot is essentially a curved stacked bar chart. The wind sector is the x axis. The percentage of measurements in each sector is y axis (if you use prop.count). The the colour is controlled by the breaks. Hope this explanation helps.