Control Visual Impact with the Alpha Channel
Using the alpha channel to control translucency can help emphasize the most relevant data on a graphical display. A good example is an election map. A common frustration with national election maps is the visual impact of land area rather than votes totals. Of course all electoral districts should be represented accurately with respect to geography. But the result is that large land areas with relatively low vote density dominate the visual message while having a much lower impact on vote totals. By manipulating the alpha channel such that areas with low vote density have a high transparency, the visual impact of area can be reduced in favor of vote totals.
The attached map was created based on vote data by county for the 2016 presidential election. The red channel was set as a gradient based on the republican vote percentage. The blue channel was set as a gradient based on the democrat vote percentage. The alpha channel was set as a gradient based on vote density (votes per unit area). Since the highest vote densities are a few very small areas far above the standard deviation, full translucency for the alpha gradient was set for a vote density below the maximum. The lower bound for the alpha gradient was set at 20% translucency for the minimum vote density so that all areas have some color density.
The result is a map that clearly shows how and where votes were cast. Even though blue comprises less area than red, the blue areas have greater visual impact because they tend to be more translucent.