def _format_warning(message, category, filename, lineno, line=None):
    """Simple format for warnings issued by ProPlot. See the
    `internal warning call signature <https://docs.python.org/3/library/warnings.html#warnings.showwarning>`__
    and the `default warning source code <https://github.com/python/cpython/blob/master/Lib/warnings.py>`__."""
    return f'{filename}:{lineno}: ProPlotWarning: {message}\n'
