Returns the description used as the plot's text alternative for assistive
technology. If labs() set an explicit alt, that string is returned
verbatim; otherwise vellumplot generates a prose summary from the specification —
the chart type, what each axis / colour / size encodes, the number of
observations, and any faceting.
Arguments
- x
A PlotSpec or a plot composition.
Details
render_plot() (and any compile through vellum::as_vellum_scene()) passes
this text to the scene's description, so the exported SVG carries it in
<desc> (with role="img") and the exported PDF tags the chart as a
Figure whose Alt is this text. The plot title (from labs(title=))
becomes the accessible name. See the Accessibility article.
Examples
p <- vplot(mtcars) |> mark_point(x = wt, y = mpg, color = hp)
plot_alt(p)
#> [1] "A scatter plot. It plots mpg (vertical axis) against wt (horizontal axis), where colour shows hp. Based on 32 observations."
plot_alt(labs(p, alt = "Heavier cars get fewer miles per gallon."))
#> [1] "Heavier cars get fewer miles per gallon."