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")

For work going on, use a progress bar.

Accessibility

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