func (b horizQuartPlot) Plot(c draw.Canvas, plt *plot.Plot) {
	trX, trY := plt.Transforms(&c)
	y := trY(b.Location)
	if !c.ContainsY(y) {
		return
	}
	y += b.Offset

	med := draw.Point{X: trX(b.Median), Y: y}
	q1 := trX(b.Quartile1)
	q3 := trX(b.Quartile3)
	aLow := trX(b.AdjLow)
	aHigh := trX(b.AdjHigh)

	c.StrokeLine2(b.WhiskerStyle, aHigh, y, q3, y)
	if c.ContainsX(med.X) {
		c.DrawGlyphNoClip(b.MedianStyle, med)
	}
	c.StrokeLine2(b.WhiskerStyle, aLow, y, q1, y)

	ostyle := b.MedianStyle
	ostyle.Radius = b.MedianStyle.Radius / 2
	for _, out := range b.Outside {
		x := trX(b.Value(out))
		if c.ContainsX(x) {
			c.DrawGlyphNoClip(ostyle, draw.Point{X: x, Y: y})
		}
	}
}

// AddBoxPlots adds box plot plotters to a plot and
// sets the X axis of the plot to be nominal.
// The variadic arguments must be either strings
// or plotter.Valuers.  Each valuer adds a box plot
// to the plot at the X location corresponding to
// the number of box plots added before it.  If a
// plotter.Valuer is immediately preceeded by a
// string then the string value is used to label the
// tick mark for the box plot's X location.
//
// If an error occurs then none of the plotters are added
// to the plot, and the error is returned.
func AddBoxPlots(plt *plot.Plot, width vg.Length, vs ...interface{}) error {
	var ps []plot.Plotter
	var names []string
	name := ""
	for _, v := range vs {
		switch t := v.(type) {
		case string:
			name = t

		case plotter.Valuer:
			b, err := plotter.NewBoxPlot(width, float64(len(names)), t)
			if err != nil {
				return err
			}
			ps = append(ps, b)
			names = append(names, name)
			name = ""

		default:
			panic(fmt.Sprintf("AddBoxPlots handles strings and plotter.Valuers, got %T", t))
		}
	}
	plt.Add(ps...)
	plt.NominalX(names...)
	return nil
}

// AddLines adds Line plotters to a plot.
// The variadic arguments must be either strings
// or plotter.XYers.  Each plotter.XYer is added to
// the plot using the next color and dashes
// shape via the Color and Dashes functions.
// If a plotter.XYer is immediately preceeded by
// a string then a legend entry is added to the plot
// using the string as the name.
//
// If an error occurs then none of the plotters are added
// to the plot, and the error is returned.
func AddLines(plt *plot.Plot, vs ...interface{}) error {
	var ps []plot.Plotter
	names := make(map[*plotter.Line]string)
	name := ""
	var i int
	for _, v := range vs {
		switch t := v.(type) {
		case string:
			name = t

		case plotter.XYer:
			l, err := plotter.NewLine(t)
			if err != nil {
				return err
			}
			l.Color = Color(i)
			l.Dashes = Dashes(i)
			i++
			ps = append(ps, l)
			if name != "" {
				names[l] = name
				name = ""
			}

		default:
			panic(fmt.Sprintf("AddLines handles strings and plotter.XYers, got %T", t))
		}
	}
	plt.Add(ps...)
	for p, n := range names {
		plt.Legend.Add(n, p)
	}
	return nil
}

// Plot implements the plot.Plotter interface.
func (g *Grid) Plot(c draw.Canvas, plt *plot.Plot) {
	trX, trY := plt.Transforms(&c)

	if g.Vertical.Color == nil {
		goto horiz
	}
	for _, tk := range plt.X.Tick.Marker.Ticks(plt.X.Min, plt.X.Max) {
		if tk.IsMinor() {
			continue
		}
		x := trX(tk.Value)
		c.StrokeLine2(g.Vertical, x, c.Min.Y, x, c.Min.Y+c.Size().Y)
	}

horiz:
	if g.Horizontal.Color == nil {
		return
	}
	for _, tk := range plt.Y.Tick.Marker.Ticks(plt.Y.Min, plt.Y.Max) {
		if tk.IsMinor() {
			continue
		}
		y := trY(tk.Value)
		c.StrokeLine2(g.Horizontal, c.Min.X, y, c.Min.X+c.Size().X, y)
	}
}

// AddScatters adds Scatter plotters to a plot.
// The variadic arguments must be either strings
// or plotter.XYers.  Each plotter.XYer is added to
// the plot using the next color, and glyph shape
// via the Color and Shape functions. If a
// plotter.XYer is immediately preceeded by
// a string then a legend entry is added to the plot
// using the string as the name.
//
// If an error occurs then none of the plotters are added
// to the plot, and the error is returned.
func AddScatters(plt *plot.Plot, vs ...interface{}) error {
	var ps []plot.Plotter
	names := make(map[*plotter.Scatter]string)
	name := ""
	var i int
	for _, v := range vs {
		switch t := v.(type) {
		case string:
			name = t

		case plotter.XYer:
			s, err := plotter.NewScatter(t)
			if err != nil {
				return err
			}
			s.Color = Color(i)
			s.Shape = Shape(i)
			i++
			ps = append(ps, s)
			if name != "" {
				names[s] = name
				name = ""
			}

		default:
			panic(fmt.Sprintf("AddScatters handles strings and plotter.XYers, got %T", t))
		}
	}
	plt.Add(ps...)
	for p, n := range names {
		plt.Legend.Add(n, p)
	}
	return nil
}

// Plot implements the Plot method of the plot.Plotter interface.
func (h *Contour) Plot(c draw.Canvas, plt *plot.Plot) {
	if naive {
		h.naivePlot(c, plt)
		return
	}

	var pal []color.Color
	if h.Palette != nil {
		pal = h.Palette.Colors()
	}

	trX, trY := plt.Transforms(&c)

	// Collate contour paths and draw them.
	//
	// The alternative naive approach is to draw each line segment as
	// conrec returns it. The integrated path approach allows graphical
	// optimisations and is necessary for contour fill shading.
	cp := contourPaths(h.GridXYZ, h.Levels, trX, trY)

	// ps is a palette scaling factor to scale the palette uniformly
	// across the given levels. This enables a discordance between the
	// number of colours and the number of levels. Sorting is not
	// necessary since contourPaths sorts the levels as a side effect.
	ps := float64(len(pal)-1) / (h.Levels[len(h.Levels)-1] - h.Levels[0])
	if len(h.Levels) == 1 {
		ps = 0
	}

	for i, z := range h.Levels {
		if math.IsNaN(z) {
			continue
		}
		for _, pa := range cp[z] {
			if isLoop(pa) {
				pa.Close()
			}

			style := h.LineStyles[i%len(h.LineStyles)]
			var col color.Color
			switch {
			case z < h.Min:
				col = h.Underflow
			case z > h.Max:
				col = h.Overflow
			case len(pal) == 0:
				col = style.Color
			default:
				col = pal[int((z-h.Levels[0])*ps+0.5)] // Apply palette scaling.
			}
			if col != nil && style.Width != 0 {
				c.SetLineStyle(style)
				c.SetColor(col)
				c.Stroke(pa)
			}
		}
	}
}

// Plot implements the plot.Plotter interface.
func (b *BarChart) Plot(c draw.Canvas, plt *plot.Plot) {
	trCat, trVal := plt.Transforms(&c)
	if b.Horizontal {
		trCat, trVal = trVal, trCat
	}

	for i, ht := range b.Values {
		catVal := b.XMin + float64(i)
		catMin := trCat(float64(catVal))
		if !b.Horizontal {
			if !c.ContainsX(catMin) {
				continue
			}
		} else {
			if !c.ContainsY(catMin) {
				continue
			}
		}
		catMin = catMin - b.Width/2 + b.Offset
		catMax := catMin + b.Width
		bottom := b.stackedOn.BarHeight(i)
		valMin := trVal(bottom)
		valMax := trVal(bottom + ht)

		var pts []draw.Point
		var poly []draw.Point
		if !b.Horizontal {
			pts = []draw.Point{
				{catMin, valMin},
				{catMin, valMax},
				{catMax, valMax},
				{catMax, valMin},
			}
			poly = c.ClipPolygonY(pts)
		} else {
			pts = []draw.Point{
				{valMin, catMin},
				{valMin, catMax},
				{valMax, catMax},
				{valMax, catMin},
			}
			poly = c.ClipPolygonX(pts)
		}
		c.FillPolygon(b.Color, poly)

		var outline [][]draw.Point
		if !b.Horizontal {
			pts = append(pts, draw.Point{X: catMin, Y: valMin})
			outline = c.ClipLinesY(pts)
		} else {
			pts = append(pts, draw.Point{X: valMin, Y: catMin})
			outline = c.ClipLinesX(pts)
		}
		c.StrokeLines(b.LineStyle, outline...)
	}
}

// Plot implements the Plotter interface, drawing a line
// that connects each point in the Line.
func (f *Function) Plot(c draw.Canvas, p *plot.Plot) {
	trX, trY := p.Transforms(&c)

	d := (p.X.Max - p.X.Min) / float64(f.Samples-1)
	line := make([]draw.Point, f.Samples)
	for i := range line {
		x := p.X.Min + float64(i)*d
		line[i].X = trX(x)
		line[i].Y = trY(f.F(x))
	}
	c.StrokeLines(f.LineStyle, c.ClipLinesXY(line)...)
}

// Plot implements the Plotter interface, drawing labels.
func (e *XErrorBars) Plot(c draw.Canvas, p *plot.Plot) {
	trX, trY := p.Transforms(&c)
	for i, err := range e.XErrors {
		y := trY(e.XYs[i].Y)
		xlow := trX(e.XYs[i].X - math.Abs(err.Low))
		xhigh := trX(e.XYs[i].X + math.Abs(err.High))

		bar := c.ClipLinesX([]draw.Point{{xlow, y}, {xhigh, y}})
		c.StrokeLines(e.LineStyle, bar...)
		e.drawCap(&c, xlow, y)
		e.drawCap(&c, xhigh, y)
	}
}

// Plot implements the Plotter interface, drawing labels.
func (l *Labels) Plot(c draw.Canvas, p *plot.Plot) {
	trX, trY := p.Transforms(&c)
	for i, label := range l.Labels {
		x := trX(l.XYs[i].X)
		y := trY(l.XYs[i].Y)
		if !c.Contains(draw.Point{X: x, Y: y}) {
			continue
		}
		x += l.XOffset
		y += l.YOffset
		c.FillText(l.TextStyle, x, y, l.XAlign, l.YAlign, label)
	}
}

func (g GlyphBoxes) Plot(c draw.Canvas, plt *plot.Plot) {
	for _, b := range plt.GlyphBoxes(plt) {
		x := c.X(b.X) + b.Rectangle.Min.X
		y := c.Y(b.Y) + b.Rectangle.Min.Y
		c.StrokeLines(g.LineStyle, []draw.Point{
			{x, y},
			{x + b.Rectangle.Size().X, y},
			{x + b.Rectangle.Size().X, y + b.Rectangle.Size().Y},
			{x, y + b.Rectangle.Size().Y},
			{x, y},
		})
	}
}

// Plot draws the BoxPlot on Canvas c and Plot plt.
func (b *BoxPlot) Plot(c draw.Canvas, plt *plot.Plot) {
	if b.Horizontal {
		b := &horizBoxPlot{b}
		b.Plot(c, plt)
		return
	}

	trX, trY := plt.Transforms(&c)
	x := trX(b.Location)
	if !c.ContainsX(x) {
		return
	}
	x += b.Offset

	med := trY(b.Median)
	q1 := trY(b.Quartile1)
	q3 := trY(b.Quartile3)
	aLow := trY(b.AdjLow)
	aHigh := trY(b.AdjHigh)

	box := c.ClipLinesY([]draw.Point{
		{x - b.Width/2, q1},
		{x - b.Width/2, q3},
		{x + b.Width/2, q3},
		{x + b.Width/2, q1},
		{x - b.Width/2 - b.BoxStyle.Width/2, q1},
	})
	c.StrokeLines(b.BoxStyle, box...)

	medLine := c.ClipLinesY([]draw.Point{
		{x - b.Width/2, med},
		{x + b.Width/2, med},
	})
	c.StrokeLines(b.MedianStyle, medLine...)

	cap := b.CapWidth / 2
	whisks := c.ClipLinesY([]draw.Point{{x, q3}, {x, aHigh}},
		[]draw.Point{{x - cap, aHigh}, {x + cap, aHigh}},
		[]draw.Point{{x, q1}, {x, aLow}},
		[]draw.Point{{x - cap, aLow}, {x + cap, aLow}})
	c.StrokeLines(b.WhiskerStyle, whisks...)

	for _, out := range b.Outside {
		y := trY(b.Value(out))
		if c.ContainsY(y) {
			c.DrawGlyphNoClip(b.GlyphStyle, draw.Point{X: x, Y: y})
		}
	}
}

// AddYErrorBars adds YErrorBars to a plot.
// The variadic arguments must be
// of type plotter.XYer, and plotter.YErrorer.
// Each errorer is added to the plot the color from
// the Colors function corresponding to its position
// in the argument list.
//
// If an error occurs then none of the plotters are added
// to the plot, and the error is returned.
func AddYErrorBars(plt *plot.Plot, es ...interface {
	plotter.XYer
	plotter.YErrorer
}) error {
	var ps []plot.Plotter
	for i, e := range es {
		bars, err := plotter.NewYErrorBars(e)
		if err != nil {
			return err
		}
		bars.Color = Color(i)
		ps = append(ps, bars)
	}
	plt.Add(ps...)
	return nil
}

// Plot implements the Plotter interface, drawing a line
// that connects each point in the Line.
func (h *Histogram) Plot(c draw.Canvas, p *plot.Plot) {
	trX, trY := p.Transforms(&c)

	for _, bin := range h.Bins {
		pts := []draw.Point{
			{trX(bin.Min), trY(0)},
			{trX(bin.Max), trY(0)},
			{trX(bin.Max), trY(bin.Weight)},
			{trX(bin.Min), trY(bin.Weight)},
		}
		if h.FillColor != nil {
			c.FillPolygon(h.FillColor, c.ClipPolygonXY(pts))
		}
		pts = append(pts, draw.Point{trX(bin.Min), trY(0)})
		c.StrokeLines(h.LineStyle, c.ClipLinesXY(pts)...)
	}
}

// AddStackedAreaPlots adds stacked area plot plotters to a plot.
// The variadic arguments must be either strings
// or plotter.Valuers.  Each valuer adds a stacked area
// plot to the plot below the stacked area plots added
// before it.  If a plotter.Valuer is immediately
// preceeded by a string then the string value is used to
// label the legend.
// Plots should be added in order of tallest to shortest,
// because they will be drawn in the order they are added
// (i.e. later plots will be painted over earlier plots).
//
// If an error occurs then none of the plotters are added
// to the plot, and the error is returned.
func AddStackedAreaPlots(plt *plot.Plot, xs plotter.Valuer, vs ...interface{}) error {
	var ps []plot.Plotter
	names := make(map[*plotter.Line]string)
	name := ""
	var i int

	for _, v := range vs {
		switch t := v.(type) {
		case string:
			name = t

		case plotter.Valuer:
			if xs.Len() != t.Len() {
				return errors.New("X/Y length mismatch")
			}

			// Make a line plotter and set its style.
			l, err := plotter.NewLine(combineXYs{xs: xs, ys: t})
			if err != nil {
				return err
			}

			l.LineStyle.Width = vg.Points(0)
			color := Color(i)
			i++
			l.ShadeColor = &color

			ps = append(ps, l)

			if name != "" {
				names[l] = name
				name = ""
			}

		default:
			panic(fmt.Sprintf("AddStackedAreaPlots handles strings and plotter.Valuers, got %T", t))
		}
	}

	plt.Add(ps...)
	for p, n := range names {
		plt.Legend.Add(n, p)
	}

	return nil
}

// AddErrorBars adds XErrorBars and YErrorBars
// to a plot.  The variadic arguments must be
// of type plotter.XYer, and must be either a
// plotter.XErrorer, plotter.YErrorer, or both.
// Each errorer is added to the plot the color from
// the Colors function corresponding to its position
// in the argument list.
//
// If an error occurs then none of the plotters are added
// to the plot, and the error is returned.
func AddErrorBars(plt *plot.Plot, vs ...interface{}) error {
	var ps []plot.Plotter
	for i, v := range vs {
		added := false

		if xerr, ok := v.(interface {
			plotter.XYer
			plotter.XErrorer
		}); ok {
			e, err := plotter.NewXErrorBars(xerr)
			if err != nil {
				return err
			}
			e.Color = Color(i)
			ps = append(ps, e)
			added = true
		}

		if yerr, ok := v.(interface {
			plotter.XYer
			plotter.YErrorer
		}); ok {
			e, err := plotter.NewYErrorBars(yerr)
			if err != nil {
				return err
			}
			e.Color = Color(i)
			ps = append(ps, e)
			added = true
		}

		if added {
			continue
		}
		panic(fmt.Sprintf("AddErrorBars expects plotter.XErrorer or plotter.YErrorer, got %T", v))
	}
	plt.Add(ps...)
	return nil
}

// Plot implements the Plot method of the plot.Plotter interface.
func (bs *Bubbles) Plot(c draw.Canvas, plt *plot.Plot) {
	trX, trY := plt.Transforms(&c)

	c.SetColor(bs.Color)

	for _, d := range bs.XYZs {
		x := trX(d.X)
		y := trY(d.Y)
		if !c.Contains(draw.Point{x, y}) {
			continue
		}

		rad := bs.radius(d.Z)

		// draw a circle centered at x, y
		var p vg.Path
		p.Move(x+rad, y)
		p.Arc(x, y, rad, 0, 2*math.Pi)
		p.Close()
		c.Fill(p)
	}
}

// Plot draws the Line, implementing the plot.Plotter
// interface.
func (pts *Line) Plot(c draw.Canvas, plt *plot.Plot) {
	trX, trY := plt.Transforms(&c)
	ps := make([]draw.Point, len(pts.XYs))

	for i, p := range pts.XYs {
		ps[i].X = trX(p.X)
		ps[i].Y = trY(p.Y)
	}

	if pts.ShadeColor != nil && len(ps) > 0 {
		c.SetColor(*pts.ShadeColor)
		minY := trY(plt.Y.Min)
		var pa vg.Path
		pa.Move(ps[0].X, minY)
		for i := range pts.XYs {
			pa.Line(ps[i].X, ps[i].Y)
		}
		pa.Line(ps[len(pts.XYs)-1].X, minY)
		pa.Close()
		c.Fill(pa)
	}

	c.StrokeLines(pts.LineStyle, c.ClipLinesXY(ps)...)
}

// AddLinePoints adds Line and Scatter plotters to a
// plot.  The variadic arguments must be either strings
// or plotter.XYers.  Each plotter.XYer is added to
// the plot using the next color, dashes, and glyph
// shape via the Color, Dashes, and Shape functions.
// If a plotter.XYer is immediately preceeded by
// a string then a legend entry is added to the plot
// using the string as the name.
//
// If an error occurs then none of the plotters are added
// to the plot, and the error is returned.
func AddLinePoints(plt *plot.Plot, vs ...interface{}) error {
	var ps []plot.Plotter
	names := make(map[[2]plot.Thumbnailer]string)
	name := ""
	var i int
	for _, v := range vs {
		switch t := v.(type) {
		case string:
			name = t

		case plotter.XYer:
			l, s, err := plotter.NewLinePoints(t)
			if err != nil {
				return err
			}
			l.Color = Color(i)
			l.Dashes = Dashes(i)
			s.Color = Color(i)
			s.Shape = Shape(i)
			i++
			ps = append(ps, l, s)
			if name != "" {
				names[[2]plot.Thumbnailer{l, s}] = name
				name = ""
			}

		default:
			panic(fmt.Sprintf("AddLinePoints handles strings and plotter.XYers, got %T", t))
		}
	}
	plt.Add(ps...)
	for ps, n := range names {
		plt.Legend.Add(n, ps[0], ps[1])
	}
	return nil
}

// Plot implements the Plot method of the plot.Plotter interface.
func (h *HeatMap) Plot(c draw.Canvas, plt *plot.Plot) {
	pal := h.Palette.Colors()
	if len(pal) == 0 {
		panic("heatmap: empty palette")
	}
	// ps scales the palette uniformly across the data range.
	ps := float64(len(pal)-1) / (h.Max - h.Min)

	trX, trY := plt.Transforms(&c)

	var pa vg.Path
	cols, rows := h.GridXYZ.Dims()
	for i := 0; i < cols; i++ {

		var right, left float64
		switch i {
		case 0:
			right = (h.GridXYZ.X(i+1) - h.GridXYZ.X(i)) / 2
			left = -right
		case cols - 1:
			right = (h.GridXYZ.X(i) - h.GridXYZ.X(i-1)) / 2
			left = -right
		default:
			right = (h.GridXYZ.X(i+1) - h.GridXYZ.X(i)) / 2
			left = -(h.GridXYZ.X(i) - h.GridXYZ.X(i-1)) / 2
		}

		for j := 0; j < rows; j++ {
			v := h.GridXYZ.Z(i, j)
			if math.IsNaN(v) || math.IsInf(v, 0) {
				continue
			}

			pa = pa[:0]

			var up, down float64
			switch j {
			case 0:
				up = (h.GridXYZ.Y(j+1) - h.GridXYZ.Y(j)) / 2
				down = -up
			case rows - 1:
				up = (h.GridXYZ.Y(j) - h.GridXYZ.Y(j-1)) / 2
				down = -up
			default:
				up = (h.GridXYZ.Y(j+1) - h.GridXYZ.Y(j)) / 2
				down = -(h.GridXYZ.Y(j) - h.GridXYZ.Y(j-1)) / 2
			}

			x, y := trX(h.GridXYZ.X(i)+left), trY(h.GridXYZ.Y(j)+down)
			dx, dy := trX(h.GridXYZ.X(i)+right), trY(h.GridXYZ.Y(j)+up)

			if !c.Contains(draw.Point{x, y}) || !c.Contains(draw.Point{dx, dy}) {
				continue
			}

			pa.Move(x, y)
			pa.Line(dx, y)
			pa.Line(dx, dy)
			pa.Line(x, dy)
			pa.Close()

			var col color.Color
			switch {
			case v < h.Min:
				col = h.Underflow
			case v > h.Max:
				col = h.Overflow
			default:
				col = pal[int((v-h.Min)*ps+0.5)] // Apply palette scaling.
			}
			if col != nil {
				c.SetColor(col)
				c.Fill(pa)
			}
		}
	}
}