[BUG]: Bar traces draw in O(n²) time since v4.0.0 (getZeroBarDir runs once per bar) #8157

Description

Since v4.0.0, drawing a bar trace takes O(n²) time whenever all bar values have the same sign, which is the common case for counts, totals, and revenue. 60k bars take ~3.7 s in v4.x versus ~0.3 s in v3.7.0, and 100k bars freeze the page for close to a minute (measured in an app that embeds plotly.js 4.1.2).

The cause is in src/traces/bar/plot.js. Since #7872, appendBarText() calls getZeroBarDir(cd, isHorizontal, xa, ya) for every bar. getZeroBarDir loops over all of cd and only returns early once it has seen both a positive and a negative value:

function appendBarText(gd, plotinfo, bar, cd, i, x0, x1, y0, y1, r, overhead, opts, makeOnCompleteCallback) {
    ...
    var zeroBarDir = getZeroBarDir(cd, isHorizontal, xa, ya);   // O(n), runs once per bar

    var text = getText(fullLayout, cd, i, xa, ya);
    textPosition = getTextPosition(trace, i);
    ...
    if (!text || textPosition === 'none' || ...) {
        bar.select('text').remove();
        return;
    }

function getZeroBarDir(cd, isHorizontal, xa, ya) {
    var hasPositive = false;
    var hasNegative = false;

    for (var i = 0; i < cd.length; i++) {
        var s = cd[i].s;
        if (s > 0) {
            hasPositive = true;
        } else if (s < 0) {
            hasNegative = true;
        }
        if (hasPositive && hasNegative) {
            return 0;
        }
    }
    ...
}

The call runs before the !text || textPosition === 'none' early return. So traces without any text, and traces with textposition: 'none', pay the quadratic cost too. Mixed-sign data exits the loop almost immediately, which is why it stays fast.

A Chrome CPU profile of a 30k-bar newPlot attributes 2.6 s of the 3.4 s long task to this function.

Screenshots/Video

N/A (performance). Plotly.newPlot timings for a single bar trace in headless Chromium 156 on an Apple M4 (macOS). Each cell is one run in a fresh page:

Doubling the bar count roughly quadruples the time on 4.x, while 3.7.0 and mixed-sign data scale linearly.

Steps to reproduce

Open this page in Chrome and look at the console. Switching the version in the script URL to 3.7.0, or making some y values negative, makes it fast again.

<!doctype html>
<html>
<head>
  <meta charset="utf-8">
  <script src="https://cdn.jsdelivr.net/npm/plotly.js-dist-min@4.2.0/plotly.min.js"></script>
</head>
<body>
  <div id="plot" style="width:900px;height:450px"></div>
  <script>
    const n = 60000;
    const x = Array.from({ length: n }, (_, i) => i);
    const y = Array.from({ length: n }, (_, i) => 1 + ((i * 7919) % 100)); // all positive
    const t0 = performance.now();
    Plotly.newPlot('plot', [{ type: 'bar', x, y }]).then(() =>
      console.log(`newPlot: ${Math.round(performance.now() - t0)} ms`)
    );
  </script>
</body>
</html>

The same happens from Python with px.bar(x=np.arange(60_000), y=np.random.rand(60_000)).show(), since plotly.py 7.1.0 bundles plotly.js 4.1.1.

Notes

• Introduced by fix(bar): prevent outside text labels from overlapping tilted axis ticks #7872 ("fix(bar): prevent outside text labels from overlapping tilted axis ticks"), commits 6b1363e / f291d70 / 79c20f8. It ships in v4.0.0 and is still present in v4.2.0 and main.
• Suggested fix: compute the zero-bar direction once per trace and pass it into appendBarText. For example, compute it in plotBars before iterating the bars and pass it through opts, or cache it on cd[0]. Moving the call below the !text / textposition === 'none' early return would also help traces without text, but traces with text would remain quadratic.
• Workaround: none from the public API, since textposition: 'none' does not avoid the call.
• Found while benchmarking chart rendering in Streamlit, which bundles plotly.js 4.1.2 for st.plotly_chart.