LED I-V curve

// An LED's I-V curve.
//
// A bench supply set to 2.0 V straight across a red LED and a 10 ohm sense
// resistor. No current-limiting resistor worth the name: the point is to
// see how the LED itself decides its current.
//
// An LED is a diode, and the engine models it as one: a Shockley junction
// with 12 ohm of bulk resistance (docs/help/parts/led.md). So its current is
// not a fixed "2 V drop". Well below the knee it passes almost nothing, and
// there every 0.1 V more multiplies the current. Past the knee the junction
// is wide open and what limits the current is resistance: the LED's own
// 12 ohm plus the 10 ohm sense resistor, so the curve straightens into a
// line of about 45 mA a volt.
//
// Draw the curve:
//
//   1. Analysis... in the more menu.
//   2. DC sweep: step vcc1's voltage from 1.0 V to 2.2 V in 0.02 V steps.
//   3. Watch the net between the LED and the sense resistor.
//
// The plot is the voltage across 10 ohm, so every 0.1 V on it is 10 mA
// through the LED. Download CSV for the LED's own current at every point.
// The x axis is the supply, which is the LED's voltage plus the little the
// sense resistor drops (0.2 V at 20 mA).
//
// At the 2.0 V the supply starts at, about 20 mA flows, which is the most a
// small LED is meant to carry, and the meter reads about 0.2 V. Click vcc1
// and try 2.5 V by hand: well over 30 mA, from half a volt more. That is
// why an LED always gets a series resistor.