func (s *Sankey) StockRange(label string, category int) (min, max float64, err error) {
	stk, ok := s.stocks[category][label]
	if !ok {
		return 0, 0, fmt.Errorf("plotter: sankey diagram does not contain stock with label=%s and category=%d", label, category)
	}
	return stk.min, stk.max, nil
}
StockRange returns the minimum and maximum value on the value axis
for the stock with the specified label and category.
package plotter
import (
	"fmt"
	"image/color"
	"math"
	"sort"
	"gonum.org/v1/plot"
	"gonum.org/v1/plot/tools/bezier"
	"gonum.org/v1/plot/vg"
	"gonum.org/v1/plot/vg/draw"
)
type Sankey struct {
	Color color.Color
	StockBarWidth vg.Length
	LineStyle draw.LineStyle
	TextStyle draw.TextStyle
	flows []Flow
	FlowStyle func(group string) (color.Color, draw.LineStyle)
	StockStyle func(label string, category int) (lbl string, ts draw.TextStyle, xOff, yOff vg.Length, c color.Color, ls draw.LineStyle)
	stocks map[int]map[string]*stock
}