code: func (f *Fpdf) RadialGradient(x, y, w, h float64, r1, g1, b1, r2, g2, b2 int, x1, y1, x2, y2, r float64) {
  	f.gradientClipStart(x, y, w, h)
  	f.gradient(3, r1, g1, b1, r2, g2, b2, x1, y1, x2, y2, r)
  	f.gradientClipEnd()
  }
docs: // RadialGradient draws a rectangular area with a blending of one color to
  // another. The rectangle is of width w and height h. Its upper left corner is
  // positioned at point (x, y).
  //
  // Each color is specified with three component values, one each for red, green
  // and blue. The values range from 0 to 255. The first color is specified by
  // (r1, g1, b1) and the second color by (r2, g2, b2).
  //
  // The blending is controlled with a point and a circle, both specified with
  // normalized coordinates in which the lower left corner of the rendered
  // rectangle is position (0, 0) and the upper right corner is (1, 1). Color 1
  // begins at the origin point specified by (x1, y1). Color 2 begins at the
  // circle specified by the center point (x2, y2) and radius r. Colors are
  // gradually blended from the origin to the circle. The origin and the circle's
  // center do not necessarily have to coincide, but the origin must be within
  // the circle to avoid rendering problems.
  //
  // The LinearGradient() example demonstrates this method.