code: func (g *GridType) TickmarksExtentX(min, div float64, count int) {
  	g.xTicks, g.xPrecision = extent(min, div, count)
  	g.xm, g.xb = linearTickmark(g.xTicks, g.x, g.x+g.w)
  }
docs: // TickmarksExtentX sets the tickmarks to be shown by Grid() in the horizontal
  // dimension. count specifies number of major tickmark subdivisions to be
  // graphed. min specifies the leftmost data value. div specifies, in data
  // units, the extent of each major tickmark subdivision.
  //
  // See TickmarkContainX() for an alternative to this method to be used when
  // viewer-friendly tickmarks are to be determined automatically.