def rgba_floats_tuple(self, x):
        """
        Provides the color corresponding to value `x` in the
        form of a tuple (R,G,B,A) with float values between 0. and 1.

        """
        if x <= self.index[0]:
            return self.colors[0]
        if x >= self.index[-1]:
            return self.colors[-1]

        i = len([u for u in self.index if u < x])  # 0 < i < n.
        return tuple(self.colors[i-1])