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Buy and sell volume on every candle
A normal volume bar tells you how much traded in a candle, not who was more aggressive. Splitting that volume into buys and sells, and drawing it on the candle itself, makes the balance visible.
What the bubbles show
In Nasdaq Life's order flow chart each candle can have a green bubble above it and a red bubble below it. Green is the volume that traded as buys (the buyer took the offer), red is the volume that traded as sells (the seller hit the bid). The bubble area grows with volume, scaled to the largest candle on screen.
How a trade becomes a buy or a sell
If the data provider reports the aggressor side, that is used. Otherwise the trade price is compared with the bid and ask at that moment, and if that is not available the tick rule decides (an uptick counts as a buy, a downtick as a sell). Trades with no basis stay "unknown". The same trade from two feeds is counted once.
Patterns traders look for
- One-sided candles: a big green bubble and a tiny red one on a rising candle show buyers in control of that minute.
- Absorption: heavy selling (a big red bubble) while the candle barely falls suggests someone is buying everything offered.
- Divergence: price makes a new high while buy volume shrinks candle after candle.
- Breakout support: a breakout candle with mostly green volume reads differently from one with mixed volume.
These are ways to describe the tape, not signals that predict price.
Limits
Bubbles exist only for minutes the server actually recorded trades for; older candles stay empty instead of showing an estimate. Side classification without a reported aggressor is an estimate. Hidden and off-exchange orders can make the visible picture incomplete.