code: func (g GridType) XRange() (min, max float64) {
  	min = g.xTicks[0]
  	max = g.xTicks[len(g.xTicks)-1]
  	return
  }
docs: // XRange returns the minimum and maximum values for the current tickmark
  // sequence. These correspond to the data values of the graph's left and right
  // edges.