Data Viz for the Visually Impaired – Part 1: Partially Sighted

Data Visualisation is all about visualising data. Whether it is a chart in a report, an infographic or a dashboard– the key point is that it is visual!

Visual: ˈvɪʒjʊəl,-adjective – Relating to seeing or sight.

So can we deliver a visual representation of data to a person without perfect vision? If so, how can we provide this person the best visualisation? Is there even any reason to cater for such individuals in the first place?

Obviously we should be doing our best to be inclusive and allow as many people as possible to benefit from our visualisations. At the most basic level, this is a legal requirement in most countries and, more practically, we want our visualisations to reach their target audiences to convey the information we are trying to portray.

Industry expert Stephen Few, in an emphatic blog post, argues that there is no way of making a visualisation for someone who cannot see at all. He says whilst blind people are definitely entitled to the same information as everyone else, the method in which that information is conveyed cannot be called a data visualisation – that a data visualisation for the blind is an oxymoron.

No channel of perception other than vision can fully duplicate the contents of graphs, in the same manner that providing the musical score to a deaf person does not allow them to fully experience the aural experience. Whatever medium we choose to convey a data visualisation to the blind, be it a screen reader, a haptic device or braille, cannot be called a visualisation or a dashboard.

However, the fact that we can’t create visualisations for those without any sight doesn’t mean that we don’t need to consider the partially sighted. The WHO estimates that 3.5% of the population worldwide suffers from low vision and this figure rises to 11.5% for over 50s.

These may seem like small percentages but when publishing in a mass market the number of affected readers may reach tens of thousands. Even when publishing for a small group of stakeholders it is possible that a number of critical people will have difficulty with your visualisations if you fail to take their impaired vision into account when designing the visualisation.

If all that fails to convince you, by designing for the visually impaired you will be making viewing and understanding your visualisations easier for the fully sighted too. Even those with 20/20 vision will struggle to read small writing, or discern the difference between very similar colours. Especially when trying to do so quickly… On a small screen on their mobile device… In direct sunlight… And so on.

So, what can we do to make visualisations accessible for those with impaired vision?

There are a few basic rules for designing for the visually impaired.

1) Font – The first step is to ensure any text is easily readable by not using excessively small or fancy fonts.

It is difficult to give exact figures for this, as there are no hard and fast rules. Even the experts can’t agree, with a minimum font size of between 12 and 16 being mentioned. Font sizes in print are generally smaller than those on digital mediums, and size can even vary between different fonts set to the same pixel size. It comes down to common sense and context – the size you choose will depend on how much text and space you have, and how crucial the text is to understanding the visualisation.

You may be tied to your corporate font, but if not, try sticking to a readable font. As with font sizes, there is a great deal of debate between experts about the virtues of serif vs sans-serif with arguments both ways. Each font family has its good and bad representatives and, once again, just stick to common sense. I don’t think anybody will argue that Arial or Verdana is clearer than a font that looks like somebody’s handwriting or a medieval gothic font.


2) Contrast

Can you read this

The above image demonstrates the importance of contrast. It’s best to avoid pale colours and ensure that there is a good colour contrast between the text and its background. There are many online resources to help check the contrast between different colours. For example and

Bear in mind that different monitors or printers may render the same colour slightly differently, so something that you see as light grey on one monitor may be totally invisible on another.

If you are aware of these issues whilst designing, you are already well on the way to producing visualisations that are accessible easily to as many people as possible.

In Part 2 we discuss dataviz for the colour blind.


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