def _draw_gradient(self, alpha, color):
        """Draw the gradient and put the cursor on alpha."""
        self.delete("gradient")
        self.delete("cursor")
        del self.gradient
        width = self.winfo_width()
        height = self.winfo_height()

        bg = create_checkered_image(width, height)
        r, g, b = color
        w = width - 1.
        gradient = Image.new("RGBA", (width, height))
        for i in range(width):
            for j in range(height):
                gradient.putpixel((i, j), (r, g, b, round2(i / w * 255)))
        self.gradient = ImageTk.PhotoImage(Image.alpha_composite(bg, gradient),
                                           master=self)

        self.create_image(0, 0, anchor="nw", tags="gardient",
                          image=self.gradient)
        self.lower("gradient")

        x = alpha / 255. * width
        h, s, v = rgb_to_hsv(r, g, b)
        if v < 50:
            fill = "gray80"
        else:
            fill = 'black'
        self.create_line(x, 0, x, height, width=2, tags='cursor', fill=fill)
Draw the gradient and put the cursor on alpha.