Repainting indicators make history look better than the present

[Shadow Trader]

An indicator that repaints changes its past values when new data arrives. A signal arrow that appeared on a bar vanishes, or moves, once later bars form. On the chart, the history shows clean, perfectly timed signals. In real time, the signals flicker, appear late, and disappear.

Zig-zag type tools are the classic example, since they connect confirmed highs and lows that are only known in hindsight. Some "support and resistance" tools and many trend indicators sold commercially behave the same way.

Screenshots of repainting indicators make convincing advertisements. A backtest on one looks astonishing because it has seen the future.

The test is simple: watch the indicator live on a demo for an hour, with screenshots every few minutes. Compare the signals as they appeared with the same chart afterwards. If they differ, it repaints.

Another check is reading the code, if available, for references to future bars or recalculation of past values.

Even legitimate indicators can repaint on the current, unfinished bar. Waiting for the bar to close before acting avoids that, at the cost of a delayed entry.

Which indicators have turned out to repaint, and how long did it take to notice?