Pending orders trigger on different sides of the quote

[Amanhasnoname]

Buy limits and buy stops fill when the ask reaches the level. Sell limits and sell stops fill when the bid reaches it. That sounds obvious until a limit order does not fill despite the chart touching the level.

On a bid-based chart, a buy limit placed at the low of the previous candle may not trigger even if the chart's low touches that level, because the ask was still higher by the spread.

The reverse also holds. A sell limit at a high may fill on the chart's high, since the bid reached it. That asymmetry means buy orders seem unlucky and sell orders seem lucky on bid charts.

The fix is the same: draw both lines, and place buy orders based on the ask. Or adjust by the spread, so the order is placed a fraction higher to improve the fill probability.

For automated systems, the code should use the right price for each order type.

Has a buy limit missed a fill on a level that appeared to be touched?