Percentage price oscillator

Momentum as a percent of price: how far a fast average has pulled away from a slow one.

PPO is the MACD line expressed as a percent of price: the fast exponential average minus the slow one, divided by the slow average, times 100. Its signal is an exponential average of PPO itself, and the histogram is PPO minus that signal. Both averages seed from the first bar and use a smoothing factor of 2 divided by window plus 1, so readings stay comparable across price levels.

How to read it

The three lines sit in their own pane below the price, centred on zero, in percent of the slow average.
Above zero means the fast average is above the slow one, so recent prices are rising faster than the longer trend; below zero is the opposite.
Histogram zero crossings are the moments PPO crosses its signal line; the histogram height is the gap between them.
On SPY one-minute bars at defaults, 90 percent of PPO values fell between -0.034 and +0.033, so judge swings against that range, not against zero alone.

What it outputs

PPO: Fast average minus slow average, divided by the slow average, in percent. Above zero means the fast average is above the slow one.
Signal: An exponential average of the PPO line itself. It trails PPO, and its crosses are the line traders watch.
Histogram: PPO minus its signal. Positive while PPO is above the signal, and its size shows the gap between the two lines.

Settings

Fast: Length of the fast average in bars. Lowering it makes PPO react sooner and swing wider; raising it toward the slow length shrinks PPO toward zero and slows its crossings.
Slow: Length of the slow average in bars, the baseline PPO divides by. Raising it makes the slow line lag further, so PPO stays on one side of zero longer in a trend; lowering it makes PPO track price more closely.
Signal: Length of the signal line's smoothing in bars. Raising it makes the signal slower, so crosses come later and less often; lowering it tightens the signal, so the histogram hugs zero and flips more often.

Pitfalls

Readings are tiny on SPY one-minute bars, so small wiggles through zero are noise; a cross matters only when the histogram is clearly away from zero.
Both averages start from the first bar they receive, so PPO is biased for roughly the first hour of bars at defaults; let history load before reading it.
The signal and histogram are smoothed versions of PPO, so their crosses confirm a turn that has already started rather than call it in advance.