It is not a gauge with its arc cut down. A gauge is a dial — a circle of scale read against a short hand near the middle. A panel meter's geometry exists because the pointer is a physical arm: the pivot sits low, the arm reaches almost the full height of the case, and the scale is the arc that arm can sweep. With that geometry come the things a dial has no use for: the anti-parallax mirror band, end stops and an over-range flag, the damping of the coil, and the centre-zero movement.
min, max and value are the scale and the reading; unit and label are printed on the plate. sweepAngle sets how far the needle travels — a moving-coil movement swings about 90 to 120 degrees.
mirror draws the anti-parallax strip under the scale — the band an operator lines the needle up with its own reflection in, which is how a panel meter is read accurately at all. It costs nothing on screen and it is what makes the instrument recognisable to anybody who has used one.
centerZero marks the rest position and draws the zero line of a ± movement. Past the ends of the scale the needle pins against the stop and the case raises an over-range flag, instead of a hand sweeping on round a dial: the reading is still reported, and the instrument says it is off scale.
Push it past ±50 to pin the needle.
damping is the inertia of the coil, in milliseconds to settle. It is off by default, because a needle that lags is showing something the process is not doing, and that is the host's decision to make. The digital reading is never damped: it is the value, and the needle is the picture of it, so the instrument cannot be caught telling two stories about the same instant.
The same samples go to both. One is readable.
ranges paints the bands on the plate and the needle takes the band's colour; setpoint with showSetpoint clips the red set pointer to the scale. quality is carried by every component that extends the scale engine: a bad reading is struck through and a stale or uncertain one is dimmed.