Print it in grey and see what is left

The four displays a chart has to survive

A programme chart is authored once and read on displays the author never sees.

Display	What it does	What fails first
A laser printer	Removes all hue	Two series distinguished only by colour
A projector in a lit room	Crushes mid-tones, lifts blacks	Light greys, thin lines, subtle shading
A phone on a metered connection	Shrinks everything	Type below 9 pt, dense axis labels
A colour-blind reader	Merges red and green	The signal palette, and any red/green pair

Every one of those is the normal case in this sector, not an accessibility edge case. The handout is photocopied, the projector is old, the reader is on a phone in a field office, and the room has a window.

The greyscale test, which takes ten seconds

import matplotlib.pyplot as plt

fig.savefig("chart.png", dpi=150)
grey = plt.imread("chart.png").mean(axis=2)     # crude luminance
plt.imsave("chart-grey.png", grey, cmap="gray")

# ggsave(), then open it in any viewer and desaturate. Or just print it.

Run it on every chart before it ships. If two series became one, the chart was encoding a distinction in hue alone.

The fix is never “pick better colours”. It is to add a second channel:

Distinguish by	How
Two lines	Direct labels at the line ends, plus solid and dashed
Bars, one emphasised	Lightness contrast, not hue contrast
Categories	Small multiples, so nothing needs distinguishing
Points in a scatter	Shape as well as colour

Lightness survives greyscale by definition. The platform’s figures use a dark spruce against a mid slate, which are two distinguishable greys before they are two colours — and that is why the emphasis in every figure on this site still reads in a photocopy.

Colour vision deficiency

About 8% of men and 0.5% of women of northern European descent have some form, and the commonest kinds merge red with green.

The signal palette is the problem case and it is unavoidable, because green, amber and red are the sector’s convention and cannot simply be replaced.

SIGNAL = {"normal": "#2f8f5b", "moderate": "#d9a02b", "severe": "#c0442f"}

# Keep the hues. Add a second channel so the hue is not load-bearing.

Three ways to keep the convention and still be readable.

Order the categories. Normal, moderate, severe in that order means position carries the severity even when hue does not.

Label directly. The word “severe” on the bar costs nothing and removes the dependence entirely.

Vary the lightness deliberately. The three signal colours on this platform are not equally light: the green is darker than the amber, so they separate in greyscale as well as in hue. Check that; a palette picked for hue alone often has two colours at identical lightness, which merge into one grey.

Type, lines and the size the chart is actually read at

plt.rcParams.update({
    "font.size": 10,          # 9 pt is the floor for a printed figure
    "axes.labelsize": 10,
    "axes.titlesize": 11,
    "xtick.labelsize": 9,
    "lines.linewidth": 1.6,   # 1 pt disappears on a projector
})
fig.set_size_inches(6.5, 4)   # the width of a text column, not of a monitor

theme_set(theme_minimal(base_size = 11))

Author at the size it will be printed. A chart designed at 12 inches wide and dropped into a 6.5-inch column has half-size type, and the fix is not to scale the image — it is to redraw at the target width.

9 pt is the floor for print and 18 pt for a projected slide. The lesson decks this platform generates are 1280×720; anything below about 18 pt at that size is not read from the back of a room.

Thin lines and light greys are the first casualties of a projector. Use 1.5 pt or heavier, and do not rely on a grey lighter than about 40% for anything that carries information.

Everything that can be deleted

The strongest accessibility measure is having less on the chart.

Delete	Because
The legend, for two series	Direct labels are read in place
Gridlines, most of them	Four is enough; twelve is a texture
The chart border and top/right spines	They carry nothing
Redundant axis units on every tick	“%” on the axis label, not on each number
The title, when the caption says it	Two headings compete
Decimal places	42.9% not 42.94%; the interval is wider than the decimal

plt.rcParams.update({"axes.spines.top": False, "axes.spines.right": False})
ax.grid(axis="x", alpha=0.3)
ax.set_axisbelow(True)              # gridlines behind the bars

theme(panel.grid.minor = element_blank(), panel.border = element_blank())

Gridlines go behind the data. A gridline drawn over a bar reads as a division in the bar, which is a mark that means nothing.

Alt text, which is a caption doing a second job

Every figure on this platform carries a caption long enough to state its finding, and that caption is what a screen reader announces.

![Global acute malnutrition by commune with 95% confidence intervals. The three
highlighted communes are the only ones whose interval clears the district
median.](/figures/muac-gam-by-commune.en.svg)

“Chart of GAM by commune” describes the file, not the finding. A reader who cannot see the chart gets nothing from it.

A caption that states the finding serves four readers at once: the screen-reader user, the reader of the printed handout, the trainer working from the PowerPoint where the figure is a caption rather than a drawing, and the person skimming the report for the point.

This is why the platform requires it rather than recommending it. figures.test.ts fails a figure whose caption is shorter than twenty characters, and the reason is not tidiness.

Report it whole

Figure accessibility conventions

  All figures are readable in greyscale: emphasis is carried by lightness
  contrast rather than hue, and series are labelled directly rather than
  through a legend.

  Signal colours (green/amber/red) are used only where the categories are
  ordered by severity, so position carries the meaning independently of hue.

  Figures are drawn at the width they are printed. Minimum type size is 9 pt
  in the report and 18 pt on slides.

  Every figure carries a caption stating its finding, which is also its
  alternative text.

Four lines, and they are checkable by anyone with a printer. An accessibility statement that cannot be verified from the artefact is a claim rather than a convention.

What comes next

A figure is rarely read where it was written. The next lesson is about the caption that has to survive being pasted into a situation report by someone who did not do the analysis.