Add Unit to XAxis as a publicly visible parameter

In some cases the XAxis may have a single long title.  This can result in very few increments being shown.
In order to be more flexible for those cases this allows the XAxis Tick frequency to be able to be directly controlled.

axis.go

@@ -176,7 +176,6 @@ func (a *axisPainter) Render() (Box, error) {
 
 	unit := opt.Unit
 	if unit <= 0 {
-
 		unit = ceilFloatToInt(float64(dataCount) / float64(fitTextCount))
 		unit = chart.MaxInt(unit, opt.SplitNumber)
 		// 偶数

xaxis.go

@@ -40,7 +40,7 @@ type XAxisOption struct {
 	FontSize float64
 	// The flag for show axis, set this to *false will hide axis
 	Show *bool
-	// Number of segments that the axis is split into. Note that this number serves only as a recommendation.
+	// Number of segments that the axis is split into. Note that this number serves only as a recommendation to avoid writing overlap.
 	SplitNumber int
 	// The position of axis, it can be 'top' or 'bottom'
 	Position string
@@ -55,6 +55,8 @@ type XAxisOption struct {
 	// The offset of label
 	LabelOffset Box
 	isValueAxis bool
+	// This value overrides SplitNumber, specifying directly the frequency at which the axis is split into, higher numbers result in less ticks
+	Unit int
 }
 
 const defaultXAxisHeight = 30
@@ -90,6 +92,7 @@ func (opt *XAxisOption) ToAxisOption() AxisOption {
 		TextRotation:   opt.TextRotation,
 		LabelOffset:    opt.LabelOffset,
 		FirstAxis:      opt.FirstAxis,
+		Unit:           opt.Unit,
 	}
 	if opt.isValueAxis {
 		axisOpt.SplitLineShow = true