plotly/plotly.js · error

cannot use zsmooth: "fast": <msg>

Error message

cannot use zsmooth: "fast": <msg>

What it means

Graceful-degradation warning emitted by noZsmooth in heatmap (and contour) calc. zsmooth:'fast' only works when x and y coordinates are evenly spaced (linear scale) and there are no axis rangebreaks; when scaleIsLinear detects uneven spacing (or another precondition fails, e.g. mismatched coordinate arrays), the fast nearest-neighbor interpolation cannot be applied correctly. Plotly then silently downgrades zsmooth to false on the trace and warns instead of throwing — the message records why the downgrade happened.

Source

Thrown at src/traces/heatmap/calc.js:89

        z = dropZonBreaks(x, y, z);

        if(!isHist) {
            x = skipBreaks(x);
            y = skipBreaks(y);

            trace._x = x;
            trace._y = y;
        }
    }

    if(!isHist && (isContour || trace.connectgaps)) {
        trace._emptypoints = findEmpties(z);
        interp2d(z, trace._emptypoints);
    }

    function noZsmooth(msg) {
        zsmooth = trace._input.zsmooth = trace.zsmooth = false;
        Lib.warn('cannot use zsmooth: "fast": ' + msg);
    }

    function scaleIsLinear(s) {
        if(s.length > 1) {
            var avgdx = (s[s.length - 1] - s[0]) / (s.length - 1);
            var maxErrX = Math.abs(avgdx / 100);
            for(i = 0; i < s.length - 1; i++) {
                if(Math.abs(s[i + 1] - s[i] - avgdx) > maxErrX) {
                    return false;
                }
            }
        }
        return true;
    }

    // Check whether all brick are uniform
    trace._islinear = false;
    if(xa.type === 'log' || ya.type === 'log') {

View on GitHub (pinned to 1d090e0b5f)

Solutions

1. Supply evenly spaced x/y coordinates (constant step) to use zsmooth:'fast'.
2. Remove axis rangebreaks that make coordinate spacing irregular.
3. Use zsmooth:'best' (or default) which handles irregular spacing.
4. Ignore the warning if the automatic fallback to zsmooth:false is acceptable.

When it happens

Trigger: Thrown at src/traces/heatmap/calc.js:89 when the library encounters an invalid state.

Common situations: See trigger scenarios.

AI-assisted analysis of plotly/plotly.js@1d090e0b5f (2026-09-02). Data as JSON: /api/errors/2a5c442f37397479. Report an issue: GitHub.