feat: support to set the first axis

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)
+	}
+}