Add possibility to render charts to image.Image

Add interface Imager for ToImage method.
Implement ToImage in rasterRenderer.
Implement ToImage in Chart, BarChart and PieChart.

bar_chart.go

@@ -2,6 +2,7 @@ package chart
 
 import (
 	"errors"
+	"image"
 	"io"
 	"math"
 
@@ -132,6 +133,56 @@ func (bc BarChart) Render(rp RendererProvider, w io.Writer) error {
 	return r.Save(w)
 }
 
+// ToImage renders the chart with the given renderer to an image.
+func (bc BarChart) ToImage(r Renderer) (image.Image, error) {
+	imager, ok := r.(Imager)
+	if !ok {
+		return nil, errors.New("Renderer does not support ToImage")
+	}
+
+	if len(bc.Bars) == 0 {
+		return nil, errors.New("Please provide at least one bar.")
+	}
+
+	if bc.Font == nil {
+		defaultFont, err := GetDefaultFont()
+		if err != nil {
+			return nil, err
+		}
+		bc.defaultFont = defaultFont
+	}
+	r.SetDPI(bc.GetDPI())
+
+	bc.drawBackground(r)
+
+	var canvasBox Box
+	var yt []Tick
+	var yr Range
+	var yf ValueFormatter
+
+	canvasBox = bc.getDefaultCanvasBox()
+	yr = bc.getRanges()
+	yr = bc.setRangeDomains(canvasBox, yr)
+	yf = bc.getValueFormatters()
+
+	if bc.hasAxes() {
+		yt = bc.getAxesTicks(r, yr, yf)
+		canvasBox = bc.getAdjustedCanvasBox(r, canvasBox, yr, yt)
+		yr = bc.setRangeDomains(canvasBox, yr)
+	}
+
+	bc.drawBars(r, canvasBox, yr)
+	bc.drawXAxis(r, canvasBox)
+	bc.drawYAxis(r, canvasBox, yr, yt)
+
+	bc.drawTitle(r)
+	for _, a := range bc.Elements {
+		a(r, canvasBox, bc.styleDefaultsElements())
+	}
+
+	return imager.ToImage(), nil
+}
+
 func (bc BarChart) getRanges() Range {
 	var yrange Range
 	if bc.YAxis.Range != nil && !bc.YAxis.Range.IsZero() {

chart.go

@@ -2,6 +2,7 @@ package chart
 
 import (
 	"errors"
+	"image"
 	"io"
 	"math"
 
@@ -132,6 +133,72 @@ func (c Chart) Render(rp RendererProvider, w io.Writer) error {
 	return r.Save(w)
 }
 
+// ToImage renders the chart with the given renderer to the given io.Writer.
+func (c Chart) ToImage(r Renderer) (image.Image, error) {
+	imager, ok := r.(Imager)
+	if !ok {
+		return nil, errors.New("Renderer does not support ToImage")
+	}
+
+	if len(c.Series) == 0 {
+		return nil, errors.New("Please provide at least one series")
+	}
+	c.YAxisSecondary.AxisType = YAxisSecondary
+
+	if c.Font == nil {
+		defaultFont, err := GetDefaultFont()
+		if err != nil {
+			return nil, err
+		}
+		c.defaultFont = defaultFont
+	}
+	r.SetDPI(c.GetDPI(DefaultDPI))
+
+	c.drawBackground(r)
+
+	var xt, yt, yta []Tick
+	xr, yr, yra := c.getRanges()
+	canvasBox := c.getDefaultCanvasBox()
+	xf, yf, yfa := c.getValueFormatters()
+	xr, yr, yra = c.setRangeDomains(canvasBox, xr, yr, yra)
+
+	err := c.checkRanges(xr, yr, yra)
+	if err != nil {
+		return nil, err
+	}
+
+	if c.hasAxes() {
+		xt, yt, yta = c.getAxesTicks(r, xr, yr, yra, xf, yf, yfa)
+		canvasBox = c.getAxisAdjustedCanvasBox(r, canvasBox, xr, yr, yra, xt, yt, yta)
+		xr, yr, yra = c.setRangeDomains(canvasBox, xr, yr, yra)
+
+		// do a second pass in case things haven't settled yet.
+		xt, yt, yta = c.getAxesTicks(r, xr, yr, yra, xf, yf, yfa)
+		canvasBox = c.getAxisAdjustedCanvasBox(r, canvasBox, xr, yr, yra, xt, yt, yta)
+		xr, yr, yra = c.setRangeDomains(canvasBox, xr, yr, yra)
+	}
+
+	if c.hasAnnotationSeries() {
+		canvasBox = c.getAnnotationAdjustedCanvasBox(r, canvasBox, xr, yr, yra, xf, yf, yfa)
+		xr, yr, yra = c.setRangeDomains(canvasBox, xr, yr, yra)
+		xt, yt, yta = c.getAxesTicks(r, xr, yr, yra, xf, yf, yfa)
+	}
+
+	c.drawCanvas(r, canvasBox)
+	c.drawAxes(r, canvasBox, xr, yr, yra, xt, yt, yta)
+	for index, series := range c.Series {
+		c.drawSeries(r, canvasBox, xr, yr, yra, series, index)
+	}
+
+	c.drawTitle(r)
+
+	for _, a := range c.Elements {
+		a(r, canvasBox, c.styleDefaultsElements())
+	}
+
+	return imager.ToImage(), nil
+}
+
 func (c Chart) getRanges() (xrange, yrange, yrangeAlt Range) {
 	var minx, maxx float64 = math.MaxFloat64, -math.MaxFloat64
 	var miny, maxy float64 = math.MaxFloat64, -math.MaxFloat64

imager.go

@@ -0,0 +1,10 @@
+package chart
+
+import "image"
+
+// Imager interface is implemented by renderers that want to expose the data
+// as the image.Image interface.
+type Imager interface {
+	// ToImage returns a image.Image
+	ToImage() image.Image
+}

pie_chart.go

@@ -2,6 +2,7 @@ package chart
 
 import (
 	"errors"
+	"image"
 	"io"
 
 	"github.com/golang/freetype/truetype"
@@ -98,6 +99,42 @@ func (pc PieChart) Render(rp RendererProvider, w io.Writer) error {
 	return r.Save(w)
 }
 
+// ToImage renders the chart with the given renderer to an image.
+func (pc PieChart) ToImage(r Renderer) (image.Image, error) {
+	imager, ok := r.(Imager)
+	if !ok {
+		return nil, errors.New("Renderer does not support ToImage")
+	}
+
+	if len(pc.Values) == 0 {
+		return nil, errors.New("Please provide at least one value.")
+	}
+
+	if pc.Font == nil {
+		defaultFont, err := GetDefaultFont()
+		if err != nil {
+			return nil, err
+		}
+		pc.defaultFont = defaultFont
+	}
+	r.SetDPI(pc.GetDPI(DefaultDPI))
+
+	canvasBox := pc.getDefaultCanvasBox()
+	canvasBox = pc.getCircleAdjustedCanvasBox(canvasBox)
+
+	pc.drawBackground(r)
+	pc.drawCanvas(r, canvasBox)
+
+	finalValues := pc.finalizeValues(pc.Values)
+	pc.drawSlices(r, canvasBox, finalValues)
+	pc.drawTitle(r)
+	for _, a := range pc.Elements {
+		a(r, canvasBox, pc.styleDefaultsElements())
+	}
+
+	return imager.ToImage(), nil
+}
+
 func (pc PieChart) drawBackground(r Renderer) {
 	Draw.Box(r, Box{
 		Right:  pc.GetWidth(),

raster_renderer.go

@@ -212,3 +212,8 @@ func (rr *rasterRenderer) ClearTextRotation() {
 func (rr *rasterRenderer) Save(w io.Writer) error {
 	return png.Encode(w, rr.i)
 }
+
+// ToImage implements the Imager interface method.
+func (rr *rasterRenderer) ToImage() image.Image {
+	return rr.i
+}