matplotlib/matplotlib · error · TypeError

Incompatible X, Y inputs to {funcname}; see help({funcname})

Error message

Incompatible X, Y inputs to {funcname}; see help({funcname})

What it means

_pcolorargs (shared by pcolor/pcolormesh) broadcasts 1-D inputs to 2-D: X is repeated along rows to (Ny, Nx), Y along columns. After broadcasting, X.shape must equal Y.shape; a mismatch raises this TypeError pointing at help(funcname).

Source

Thrown at lib/matplotlib/axes/_axes.py:6516

                if np.ma.is_masked(X) or np.ma.is_masked(Y):
                    raise ValueError(
                        'x and y arguments to pcolormesh cannot have '
                        'non-finite values or be of type '
                        'numpy.ma.MaskedArray with masked values')
            nrows, ncols = C.shape[:2]
        else:
            raise _api.nargs_error(funcname, takes="1 or 3", given=len(args))

        Nx = X.shape[-1]
        Ny = Y.shape[0]
        if X.ndim != 2 or X.shape[0] == 1:
            x = X.reshape(1, Nx)
            X = x.repeat(Ny, axis=0)
        if Y.ndim != 2 or Y.shape[1] == 1:
            y = Y.reshape(Ny, 1)
            Y = y.repeat(Nx, axis=1)
        if X.shape != Y.shape:
            raise TypeError(f'Incompatible X, Y inputs to {funcname}; '
                            f'see help({funcname})')

        if shading == 'auto':
            if ncols == Nx and nrows == Ny:
                shading = 'nearest'
            else:
                shading = 'flat'

        if shading == 'flat':
            if (Nx, Ny) != (ncols + 1, nrows + 1):
                raise TypeError(f"Dimensions of C {C.shape} should"
                                f" be one smaller than X({Nx}) and Y({Ny})"
                                f" while using shading='flat'"
                                f" see help({funcname})")
        else:    # ['nearest', 'gouraud']:
            if (Nx, Ny) != (ncols, nrows):
                raise TypeError('Dimensions of C %s are incompatible with'
                                ' X (%d) and/or Y (%d); see help(%s)' % (

View on GitHub (pinned to b379c1b69e)

Solutions

1. Generate both together: X, Y = np.meshgrid(x_1d, y_1d) - or just pass the 1-D vectors: ax.pcolormesh(x_1d, y_1d, C).
2. Check X.shape == Y.shape right before plotting and transpose one (X = X.T) if a single axis is swapped.
3. Remember the convention: X.shape[-1] is Nx (columns), Y.shape[0] is Ny (rows).

Example fix

// before
X = np.meshgrid(xs, ys)[0]  # (Ny, Nx)
Y = other_ys               # wrong length
ax.pcolormesh(X, Y, C)

// after
X, Y = np.meshgrid(xs, ys)
ax.pcolormesh(X, Y, C)  # or ax.pcolormesh(xs, ys, C)

import numpy as np

def xy_compatible(X, Y):
    X, Y = np.asarray(X), np.asarray(Y)
    Nx, Ny = X.shape[-1], Y.shape[0]
    Xb = (np.repeat(X.reshape(1, Nx), Ny, axis=0)
          if (X.ndim != 2 or X.shape[0] == 1) else X)
    Yb = (np.repeat(Y.reshape(Ny, 1), Nx, axis=1)
          if (Y.ndim != 2 or Y.shape[1] == 1) else Y)
    return Xb.shape == Yb.shape

# usage: assert xy_compatible(X, Y) before pcolormesh/pcolor

When it happens

Trigger: 2-D X of shape (Ny1, Nx) paired with 1-D y of length Ny2 != Ny1; X transposed to (Nx, Ny) relative to Y; X and Y taken from two different grids after regridding.

Common situations: np.meshgrid misuse (using only one output, or mixing sparse/dense); an old X grid kept with a new Y vector after a resolution change; assuming square grids make shape bugs invisible.

Related errors

• Dimensions of C %s are incompatible with X (%d) and/or Y (%d
• Matplotlib requires access to a writable cache directory, bu
• Key {key}: {ve}
• x and y arguments to pcolormesh cannot have non-finite value
• Dimensions of C {C.shape} should be one smaller than X({Nx})

AI-assisted analysis of matplotlib/matplotlib@b379c1b69e (2026-08-21). Data as JSON: /api/errors/2f360b8400b3a2f7. Report an issue: GitHub.