Chart

A chart is an encoding before it is a picture. Every value in the dataset is turned into something the eye can measure: the height of a bar, the position of a point, the angle of a wedge, the darkness of a cell. Choosing that encoding is the whole design decision, because the eye is far better at some comparisons than others. Length along a common baseline is read almost exactly, position along a shared axis nearly as well, and angle and area are read badly enough that a pie with more than a few slices stops answering the question it was drawn for. The right chart is the one whose encoding makes the comparison the reader actually came for the easiest thing on the page.

That is also the line between this word and its neighbours. A chart plots a dataset against scales, so it carries axes, a domain, and some way of saying what each mark stands for. A gauge reports one number against a range and adds a judgement about where in that range the number sits, which is a different job with a different failure mode. A sparkline is a chart with its scales taken away so it can sit inside a line of text, trading precise reading for shape at a glance. And a data table beats all of them when the reader wants to look one figure up rather than compare a set of them, which is why a serious report usually ships both.

Colour in a chart is data, not decoration. When a hue tells one series from another it is carrying meaning, so it has to survive being printed, dimmed, or seen by a reader with a colour vision deficiency. Pick series colours from a categorical palette built to stay distinguishable, and give every series a second cue as well: a direct label at the end of the line, a marker shape, a dash pattern. The rule behind use of colour everywhere else applies here with more force, because a chart is often the only place a finding appears at all.

A chart is content, so it needs a text alternative. Useful alt text for a chart is not a description of the drawing but the finding it was drawn to show, with the figures that support it; “bar chart of revenue” tells a reader nothing the caption had not already given them. Where the underlying numbers matter, publish them, in a table beside the chart or behind a disclosure, which serves a screen reader, a copy and paste, and a reader who simply distrusts the axis. Apple’s Human Interface Guidelines argues the same case from the other end: make the chart legible before making it animated or interactive, because a reader who cannot read it will not wait for it to move.