Meter

ui.Meter creates a bar showing a value between two bounds, as AppKit's level indicator and the web's meter: in the theme's Success color, or Warning and Danger from the levels given, nil for none.

size := app.disk.Size
ui.Meter(c, app.disk.Used, 0, size, &ui.MeterLevels{Warning: 0.8 * size, Critical: 0.95 * size}).Label("Disk")

MeterLevels are the levels from which the value shows it is worse: the Warning color from Warning, the Danger color from Critical. A Critical below Warning makes low values the bad ones, as a battery's:

ui.Meter(c, app.battery, 0, 100, &ui.MeterLevels{Warning: 20, Critical: 10}).Label("Battery")

Colors

NormalColor, WarningColor and CriticalColor fill the bar in colors of your own in place of the theme's Success, Warning and Danger; those left zero keep the theme's. A storage bar in a neutral gray until it is almost full, then amber, and the theme's Danger at the end:

ui.Meter(c, used, 0, size, &ui.MeterLevels{
	Warning: 0.8 * size, Critical: 0.95 * size,
	NormalColor: ui.RGB(142, 142, 147), WarningColor: ui.RGB(255, 159, 10),
}).Label("Storage")

Levels without Warning and Critical fill it in NormalColor throughout, as a quota bar in a product's own color. Background colors the track, the theme's Border by default:

ui.Meter(c, app.used, 0, app.quota, &ui.MeterLevels{NormalColor: brand}).Background(brandTrack).Label("Backup quota")

The bar keeps its shape, its range, its direction right to left and what assistive technology reads of it.

For work going on, use a progress bar.

Accessibility

Assistive technology sees a level indicator of the value, named by its Label.