How do we see the world in colors? It all starts with the light. The light can come from the Sun, fire or a lamp sending waves of different lengths. The surfaces around us have different capacities for absorbing wavelengths, so some of the waves are absorbed, while others are reflected. The ones that are reflected can be perceived by our eyes. These waves go through the cornea and pupil and hit the retina. […]

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The best charts for color blind viewers

How do we see the world in colors? It all starts with the light. The light can come from the Sun, fire or a lamp sending waves of different lengths. The surfaces around us have different capacities for absorbing wavelengths, so some of the waves are absorbed, while others are reflected. The ones that are reflected can be perceived by our eyes. These waves go through the cornea and pupil and hit the retina. The retina consists of cones and rods. Cones are the ones that are responsible for color perception. The combination of waves that hit the cones form in our brain the perception of color.

What colors can color blind people see?

Actually, it depends on what type of cones don’t work. There are three common types: protanopia (red-blind), deuteranopia (green-blind), and tritanopia (blue-blind). There can also be an intermediate state, color weakness — when the colors can be distinguished but some of the hue details are lost. There’s also a very rare state when all the cones are not working and the person is completely color blind — it’s called complete achromatopsia. In the next picture, the simulation of what colors can color blind people see is presented.

There are around 300 million people in the world who are colorblind. About 8% of men and 0.5% of women are colorblind, so making your chart color blind safe is a reasonable thing to do.

How to make charts color blind friendly?

Use shapes and icons as an addition or alternative to color-coding. In case one needs to use colors that are not perceptible by colorblind users, a chart can be saved by using icons as an addition to color, duplicating its informational function.

Typically, it’s better to use direct labels instead of a legend — it saves the time and attention of a reader. Another advantage of direct labels is its ability to fix the usage of palettes that are not color blind friendly.

For line charts and their variations, dashed lines and lines with various stroke thicknesses can be a very helpful alternative to coloring.

Adding strokes around chart elements might also help to distinguish one element from the other if the color might look the same for color-blind users.

If the usage of color is inevitable try to use a single hue palette. It will make the chart readable for all kinds of color blindness including monochromacy (when no color can be recognized). The second option is to use a red-yellow-blue palette. It will work for all kinds of color blindness except monochromacy.

Use color for groups, not the individual categories. By doing that the number of colors will be reduced, as well as visual clutter and the possibility of color confusion.

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