def to_css(color):
    """Convert a Toyplot color to a CSS string.

    Parameters
    ----------
    color: :class:`numpy.ndarray`
        Array of RGBA values with dtype = :data:`toyplot.color.dtype`, which is converted to a CSS rgba() color.

    Returns
    -------
    css: :class:`str` containing a CSS color value.
      """
    return 'rgba(%.1f%%,%.1f%%,%.1f%%,%.3f)' % (color['r'] * 100, color['g'
        ] * 100, color['b'] * 100, color['a'])
