gridlines.

axis.go

@@ -15,5 +15,6 @@ type Axis interface {
 	GetName() string
 	GetStyle() Style
 	GetTicks(r Renderer, ra Range, vf ValueFormatter) []Tick
+	GetGridLines(ticks []Tick) []GridLine
 	Render(c *Chart, r Renderer, canvasBox Box, ra Range, ticks []Tick)
 }

chart.go

@@ -24,8 +24,9 @@ type Chart struct {
 	YAxis          YAxis
 	YAxisSecondary YAxis
 
-	Font   *truetype.Font
+	Font     *truetype.Font
-	Series []Series
+	Series   []Series
+	Elements []Renderable
 }
 
 // GetDPI returns the dpi for the chart.
@@ -100,6 +101,10 @@ func (c Chart) Render(rp RendererProvider, w io.Writer) error {
 	}
 	c.drawTitle(r)
 
+	for _, a := range c.Elements {
+		a(r, canvasBox)
+	}
+
 	return r.Save(w)
 }
 

defaults.go

@@ -82,6 +82,8 @@ var (
 	DefaultFillColor = drawing.Color{R: 0, G: 217, B: 116, A: 255}
 	// DefaultAnnotationFillColor is the default annotation background color.
 	DefaultAnnotationFillColor = drawing.Color{R: 255, G: 255, B: 255, A: 255}
+	// DefaultGridLineColor is the default grid line color.
+	DefaultGridLineColor = drawing.Color{R: 239, G: 239, B: 239, A: 255}
 )
 
 var (

grid_line.go

@@ -30,14 +30,27 @@ func (gl GridLine) Horizontal() bool {
 
 // Render renders the gridline
 func (gl GridLine) Render(r Renderer, canvasBox Box, ra Range) {
-	lineleft := canvasBox.Left
+	if gl.IsVertical {
-	lineright := canvasBox.Right
+		lineLeft := canvasBox.Left + ra.Translate(gl.Value)
-	lineheight := canvasBox.Bottom - ra.Translate(gl.Value)
+		lineBottom := canvasBox.Bottom
+		lineTop := canvasBox.Top
 
-	r.SetStrokeColor(gl.Style.GetStrokeColor(DefaultAxisColor))
+		r.SetStrokeColor(gl.Style.GetStrokeColor(DefaultAxisColor))
-	r.SetStrokeWidth(gl.Style.GetStrokeWidth(DefaultAxisLineWidth))
+		r.SetStrokeWidth(gl.Style.GetStrokeWidth(DefaultAxisLineWidth))
 
-	r.MoveTo(lineleft, lineheight)
+		r.MoveTo(lineLeft, lineBottom)
-	r.LineTo(lineright, lineheight)
+		r.LineTo(lineLeft, lineTop)
-	r.Stroke()
+		r.Stroke()
+	} else {
+		lineLeft := canvasBox.Left
+		lineRight := canvasBox.Right
+		lineHeight := canvasBox.Bottom - ra.Translate(gl.Value)
+
+		r.SetStrokeColor(gl.Style.GetStrokeColor(DefaultAxisColor))
+		r.SetStrokeWidth(gl.Style.GetStrokeWidth(DefaultAxisLineWidth))
+
+		r.MoveTo(lineLeft, lineHeight)
+		r.LineTo(lineRight, lineHeight)
+		r.Stroke()
+	}
 }

renderable.go

@@ -1,6 +1,4 @@
 package chart
 
-// Renderable is a type that can be rendered onto a chart.
+// Renderable is a function that can be called to render custom elements on the chart.
-type Renderable interface {
+type Renderable func(r Renderer, canvasBox Box)
-	Render(r Renderer, canvasBox Box, xrange, yrange Range, defaults Style)
-}

series.go

@@ -3,5 +3,5 @@ package chart
 // Series is an alias to Renderable.
 type Series interface {
 	GetYAxis() YAxisType
-	Renderable
+	Render(r Renderer, canvasBox Box, xrange, yrange Range, s Style)
 }

testserver/main.go

@@ -46,6 +46,12 @@ func chartHandler(rc *web.RequestContext) web.ControllerResult {
 			Style: chart.Style{
 				Show: true,
 			},
+			GridMajorStyle: chart.Style{
+				Show: true,
+			},
+			GridMinorStyle: chart.Style{
+				Show: true,
+			},
 		},
 		YAxis: chart.YAxis{
 			Style: chart.Style{
@@ -57,6 +63,12 @@ func chartHandler(rc *web.RequestContext) web.ControllerResult {
 					StrokeWidth: 1.0,
 				},
 			},
+			GridMajorStyle: chart.Style{
+				Show: true,
+			},
+			GridMinorStyle: chart.Style{
+				Show: true,
+			},
 		},
 		Series: []chart.Series{
 			chart.TimeSeries{

xaxis.go

@@ -12,6 +12,10 @@ type XAxis struct {
 	ValueFormatter ValueFormatter
 	Range          Range
 	Ticks          []Tick
+
+	GridLines      []GridLine
+	GridMajorStyle Style
+	GridMinorStyle Style
 }
 
 // GetName returns the name.
@@ -62,6 +66,41 @@ func (xa XAxis) getTickCount(r Renderer, ra Range, vf ValueFormatter) int {
 	return count
 }
 
+// GetGridLines returns the gridlines for the axis.
+func (xa XAxis) GetGridLines(ticks []Tick) []GridLine {
+	if len(xa.GridLines) > 0 {
+		return xa.GridLines
+	}
+	return xa.generateGridLines(ticks)
+}
+
+func (xa XAxis) generateGridLines(ticks []Tick) []GridLine {
+	var gl []GridLine
+	isMinor := false
+	minorStyle := Style{
+		StrokeColor: DefaultGridLineColor.WithAlpha(100),
+		StrokeWidth: 1.0,
+	}
+	majorStyle := Style{
+		StrokeColor: DefaultGridLineColor,
+		StrokeWidth: 1.0,
+	}
+	for _, t := range ticks {
+		s := majorStyle
+		if isMinor {
+			s = minorStyle
+		}
+		gl = append(gl, GridLine{
+			Style:      s,
+			IsMinor:    isMinor,
+			IsVertical: true,
+			Value:      t.Value,
+		})
+		isMinor = !isMinor
+	}
+	return gl
+}
+
 // Measure returns the bounds of the axis.
 func (xa XAxis) Measure(r Renderer, canvasBox Box, ra Range, ticks []Tick) Box {
 	defaultFont, _ := GetDefaultFont()
@@ -121,4 +160,13 @@ func (xa XAxis) Render(r Renderer, canvasBox Box, ra Range, ticks []Tick) {
 		r.LineTo(tx, canvasBox.Bottom+DefaultVerticalTickHeight)
 		r.Stroke()
 	}
+
+	if xa.GridMajorStyle.Show || xa.GridMinorStyle.Show {
+		for _, gl := range xa.GetGridLines(ticks) {
+			if (gl.IsMinor && xa.GridMinorStyle.Show) ||
+				(!gl.IsMinor && xa.GridMajorStyle.Show) {
+				gl.Render(r, canvasBox, ra)
+			}
+		}
+	}
 }

yaxis.go

@@ -18,6 +18,10 @@ type YAxis struct {
 	ValueFormatter ValueFormatter
 	Range          Range
 	Ticks          []Tick
+
+	GridLines      []GridLine
+	GridMajorStyle Style
+	GridMinorStyle Style
 }
 
 // GetName returns the name.
@@ -62,6 +66,40 @@ func (ya YAxis) getTickCount(r Renderer, ra Range, vf ValueFormatter) int {
 	return int(math.Ceil(float64(ra.Domain) / float64(tb.Height()+DefaultMinimumTickVerticalSpacing)))
 }
 
+// GetGridLines returns the gridlines for the axis.
+func (ya YAxis) GetGridLines(ticks []Tick) []GridLine {
+	if len(ya.GridLines) > 0 {
+		return ya.GridLines
+	}
+	return ya.generateGridLines(ticks)
+}
+
+func (ya YAxis) generateGridLines(ticks []Tick) []GridLine {
+	var gl []GridLine
+	isMinor := false
+	minorStyle := Style{
+		StrokeColor: DefaultGridLineColor.WithAlpha(100),
+		StrokeWidth: 1.0,
+	}
+	majorStyle := Style{
+		StrokeColor: DefaultGridLineColor,
+		StrokeWidth: 1.0,
+	}
+	for _, t := range ticks {
+		s := majorStyle
+		if isMinor {
+			s = minorStyle
+		}
+		gl = append(gl, GridLine{
+			Style:   s,
+			IsMinor: isMinor,
+			Value:   t.Value,
+		})
+		isMinor = !isMinor
+	}
+	return gl
+}
+
 // Measure returns the bounds of the axis.
 func (ya YAxis) Measure(r Renderer, canvasBox Box, ra Range, ticks []Tick) Box {
 	defaultFont, _ := GetDefaultFont()
@@ -159,4 +197,13 @@ func (ya YAxis) Render(r Renderer, canvasBox Box, ra Range, ticks []Tick) {
 	if ya.Zero.Style.Show {
 		ya.Zero.Render(r, canvasBox, ra)
 	}
+
+	if ya.GridMajorStyle.Show || ya.GridMinorStyle.Show {
+		for _, gl := range ya.GetGridLines(ticks) {
+			if (gl.IsMinor && ya.GridMinorStyle.Show) ||
+				(!gl.IsMinor && ya.GridMajorStyle.Show) {
+				gl.Render(r, canvasBox, ra)
+			}
+		}
+	}
 }