The Data Daily

How to write good alternative descriptions for your data visualization

Last updated: 04-06-2021

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How to write good alternative descriptions for your data visualization

Writing an alternative description only takes a couple of minutes. They are a great help if you want to make sure readers with visual impairments don't miss out on relevant content in your data visualization. This article gives you some tips on how to write good alt text for a better screen reader experience.

Alternative text, also known as alt text, is a written description that allows you to convey the meaning of visual elements, such as images, visualizations or other media, to help users with disabilities understand the content they are missing. 

This attribute is read aloud by screen readers, which is the assistive technology used by people with visual impairments.

To add an alternative description in Datawrapper, go to step 3: Visualize. In the Annotate tab, enter your description in the Alternative description for screen readers field.

To structure your text, you can also make use of HTML. Maybe you want to summarize the most crucial data points in a short table. Perhaps, there's a URL with more accessible information that you wish to link to.

Data visualization designer Amy Cesal came up with this formula on what you should include in the alt text for your graphic:

alt=" Chart type of type of data where reason for including chart"

This formula is not set in stone. You could also argue that any mention of the chart type is unnecessary: The visualization is a way of representing the data; if words are used instead to describe the data, there's no need to talk about visual elements at all. 

Depending on what makes the most sense for you and the data, and depending on organization-internal accessibility instructions, you might want to choose Amy's approach, or you might not want to mention the chart type.

When working with a chart that won't get updated, you can write a more detailed description. Let's use as an example this split bars chart:

An alternative description for this chart could be:

A split bars chart of different types of milk's (dairy and plant-based) environmental impact, where cow's milk scores significantly worst in carbon emissions, land use, and water use than the other milk alternatives. Cow's milk produces two times more carbon emissions, uses nine times more land, and twice the water that rice milk uses. Rice milk is the second most contaminating type of milk in terms of carbon emissions.

With visualizations that will likely get updated frequently, you should try to give a general description, as well as highlighting some relevant data values and trends. Make sure you update your alt text every time the highlight in your chart changes. For this example, let's use a line chart:

An alternative description for this chart could be:

A line chart of historical Bitcoin price in USD and volume of bitcoins traded from January 2017 to January 2021, where Bitcoin reached its highest price ever on January 8th, 2021, when it was traded for over $40,600. The second highest peak happened at the end of 2017, a year in which Bitcoin went from $830 to $19,300. The current value, on January 13th, is $34,700. The volume of Bitcoin traded also saw its highest peak in January 2021, topping $123 billion traded.

When writing an alt description for a map, you should avoid focusing on the visual elements and highlight the important message instead that your map conveys. Let's have a look at the choropleth map from this Weekly Chart here:

An alternative description for this chart could be:

A choropleth map of the proportion of women in national parliaments in the world in 2020, where Rwanda stands out as the country with the highest percentage of women (61%) in a national parliament, followed by Bolivia and Cuba, with a 53%. Papua New Guinea is the country that scored the worst with 0% of women in their parliament.

At the end of this description, you could also include some countries that you think are relevant to your readers.

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