Choosing how you present a specific theme is important. It can be done in very different ways. It can communicate very different things. I have presented a lot of maps to people of various backgrounds and there is one thing you can depend on. The legend is the last thing they will look at. The human is a visual animal and the legend just gets in the way. The immediate impression is the one that sticks with the regular map viewer. Take a look at three maps below displays the spread of HIV/AIDS in Ethiopia. The first looks at prevalence. The second looks at individuals. The third displays density. The use of colors to represent the data implies that HIV/AIDS is low (left map), high (middle map) and medium (right map).