feat: support to set the first axis

axis.go

@@ -63,6 +63,8 @@ type AxisOption struct {
 	StrokeWidth float64
 	// The length of the axis tick
 	TickLength int
+	// The first axis
+	FirstAxis int
 	// The margin value of label
 	LabelMargin int
 	// The font size of label
@@ -255,6 +257,7 @@ func (a *axisPainter) Render() (Box, error) {
 			Length: tickLength,
 			Unit:   unit,
 			Orient: orient,
+			First:  opt.FirstAxis,
 		})
 		p.LineStroke([]Point{
 			{
@@ -273,6 +276,7 @@ func (a *axisPainter) Render() (Box, error) {
 		Top:   labelPaddingTop,
 		Right: labelPaddingRight,
 	})).MultiText(MultiTextOption{
+		First:        opt.FirstAxis,
 		Align:        textAlign,
 		TextList:     data,
 		Orient:       orient,

examples/time_line_chart/main.go

@@ -0,0 +1,82 @@
+package main
+
+import (
+	"crypto/rand"
+	"fmt"
+	"io/ioutil"
+	"math/big"
+	"os"
+	"path/filepath"
+	"time"
+
+	"github.com/vicanso/go-charts/v2"
+)
+
+func writeFile(buf []byte) error {
+	tmpPath := "./tmp"
+	err := os.MkdirAll(tmpPath, 0700)
+	if err != nil {
+		return err
+	}
+
+	file := filepath.Join(tmpPath, "time-line-chart.png")
+	err = ioutil.WriteFile(file, buf, 0600)
+	if err != nil {
+		return err
+	}
+	return nil
+}
+
+func main() {
+	xAxisValue := []string{}
+	values := []float64{}
+	now := time.Now()
+	firstAxis := 0
+	for i := 0; i < 300; i++ {
+		// 设置首个axis为xx:00的时间点
+		if firstAxis == 0 && now.Minute() == 0 {
+			firstAxis = i
+		}
+		xAxisValue = append(xAxisValue, now.Format("15:04"))
+		now = now.Add(time.Minute)
+		value, _ := rand.Int(rand.Reader, big.NewInt(100))
+		values = append(values, float64(value.Int64()))
+	}
+	p, err := charts.LineRender(
+		[][]float64{
+			values,
+		},
+		charts.TitleTextOptionFunc("Line"),
+		charts.XAxisDataOptionFunc(xAxisValue, charts.FalseFlag()),
+		charts.LegendLabelsOptionFunc([]string{
+			"Demo",
+		}, "50"),
+		func(opt *charts.ChartOption) {
+			opt.XAxis.FirstAxis = firstAxis
+			// 必须要比计算得来的最小值更大(每60分钟)
+			opt.XAxis.SplitNumber = 60
+			opt.Legend.Padding = charts.Box{
+				Top:    5,
+				Bottom: 10,
+			}
+			opt.SymbolShow = charts.FalseFlag()
+			opt.LineStrokeWidth = 1
+			opt.ValueFormatter = func(f float64) string {
+				return fmt.Sprintf("%.0f", f)
+			}
+		},
+	)
+
+	if err != nil {
+		panic(err)
+	}
+
+	buf, err := p.Bytes()
+	if err != nil {
+		panic(err)
+	}
+	err = writeFile(buf)
+	if err != nil {
+		panic(err)
+	}
+}

painter.go

@@ -59,6 +59,8 @@ type PainterOptions struct {
 type PainterOption func(*Painter)
 
 type TicksOption struct {
+	// the first tick
+	First  int
 	Length int
 	Orient string
 	Count  int
@@ -74,6 +76,8 @@ type MultiTextOption struct {
 	// The text rotation of label
 	TextRotation float64
 	Offset       Box
+	// The first text index
+	First int
 }
 
 type GridOption struct {
@@ -616,6 +620,7 @@ func (p *Painter) Ticks(opt TicksOption) *Painter {
 		return p
 	}
 	count := opt.Count
+	first := opt.First
 	width := p.Width()
 	height := p.Height()
 	unit := 1
@@ -630,7 +635,10 @@ func (p *Painter) Ticks(opt TicksOption) *Painter {
 		values = autoDivide(width, count)
 	}
 	for index, value := range values {
-		if index%unit != 0 {
+		if index < first {
+			continue
+		}
+		if (index-first)%unit != 0 {
 			continue
 		}
 		if isVertical {
@@ -688,7 +696,10 @@ func (p *Painter) MultiText(opt MultiTextOption) *Painter {
 	isTextRotation := opt.TextRotation != 0
 	offset := opt.Offset
 	for index, text := range opt.TextList {
-		if opt.Unit != 0 && index%opt.Unit != showIndex {
+		if index < opt.First {
+			continue
+		}
+		if opt.Unit != 0 && (index-opt.First)%opt.Unit != showIndex {
 			continue
 		}
 		if isTextRotation {

xaxis.go

@@ -50,6 +50,8 @@ type XAxisOption struct {
 	FontColor Color
 	// The text rotation of label
 	TextRotation float64
+	// The first axis
+	FirstAxis int
 	// The offset of label
 	LabelOffset Box
 	isValueAxis bool
@@ -87,6 +89,7 @@ func (opt *XAxisOption) ToAxisOption() AxisOption {
 		SplitLineColor: opt.Theme.GetAxisSplitLineColor(),
 		TextRotation:   opt.TextRotation,
 		LabelOffset:    opt.LabelOffset,
+		FirstAxis:      opt.FirstAxis,
 	}
 	if opt.isValueAxis {
 		axisOpt.SplitLineShow = true