code: func (g *GridType) TickmarksExtentY(min, div float64, count int) {
  	g.yTicks, g.yPrecision = extent(min, div, count)
  	g.ym, g.yb = linearTickmark(g.yTicks, g.y+g.h, g.y)
  }
docs: // TickmarksExtentY sets the tickmarks to be shown by Grid() in the vertical
  // dimension. count specifies number of major tickmark subdivisions to be
  // graphed. min specifies the bottommost data value. div specifies, in data
  // units, the extent of each major tickmark subdivision.
  //
  // See TickmarkContainY() for an alternative to this method to be used when
  // viewer-friendly tickmarks are to be determined automatically.