Exponential trailing stop

A trailing stop that starts below entry, only moves up, and tightens as the trade gains.

Given a trade's entry price, it draws a stop-loss line that starts initial_stop_pct below entry, 2% by default. Each bar the line may only rise: it is set to a gap below price, and that gap shrinks exponentially as profit grows, halving for every 10% of gain at the default factor. The line resets to a new starting level whenever the entry input changes, and stays flat through pullbacks.

How to read it

The line sits on the price scale, over the candles, starting about 2% below the entry price.
It only steps up. Each rise lifts it to a gap below the new price, and pullbacks leave it flat.
The gap narrows as the trade gains, so a steep rally brings the line close to the candles.
The indicator draws the level only. Price closing below it is the stop being hit, but nothing is marked.

What it outputs

Trailing stop: The stop-loss level for the open trade, in price units. It starts below entry, only moves up as price rises, and resets when the entry input changes.

Settings

initial_stop_pct: Starting stop distance below entry, as a fraction (0.02 = 2%). Raising it gives the trade more room and a lower first stop; lowering it puts the stop near entry, so small dips end the trade. It also sets the gap while the trade is at or below entry.
tightening_factor: Multiplies the trailing gap for every +10% of profit. At 0.5 the gap halves per 10%; at 1 it never shrinks, so the stop trails at a fixed distance. Lower values tighten faster; above 1 the gap widens as profit grows.

Pitfalls

The stop only resets when the entry input changes. Wired to a value that moves every bar, it sits about 2% under price: SPY min, median and max all equal 0.98 times the close's.
Negative values pass the slider range but break the math. A negative tightening_factor gives complex numbers and a TypeError in max() once in profit; a negative initial_stop_pct puts the stop above entry.
The gap is a fixed fraction of entry, not a volatility measure, so it ignores how fast SPY is moving. The line is a level only; it does not exit a position or size one.