Improvements to how the X Axis is rendered

This provides two improvements to how the X Axis is rendered:
* The calculation for where a tick should exist has been improved.  It now will ensure a tick is always at both the start of the axis and the end of the axis.  This makes it clear exactly what data span is captured in the graph.
* The second improvement is how the label on the last tick is written.  It used to often get partially cut off, and with the change to ensure a tick is always at the end this could be seen more easily.  Now the last tick has it's label written to the left so that it can be fully displayed.

painter.go

@@ -615,6 +615,17 @@ func (p *Painter) TextFit(body string, x, y, width int, textAligns ...string) ch
 	return output
 }
 
+func isTick(totalRange int, numTicks int, index int) bool {
+	step := float64(totalRange-1) / float64(numTicks-1)
+	for i := 0; i < numTicks; i++ {
+		value := float64(i) * step
+		if int(value + 0.5) == index {
+			return true
+		}
+	}
+	return false
+}
+
 func (p *Painter) Ticks(opt TicksOption) *Painter {
 	if opt.Count <= 0 || opt.Length <= 0 {
 		return p
@@ -638,7 +649,7 @@ func (p *Painter) Ticks(opt TicksOption) *Painter {
 		if index < first {
 			continue
 		}
-		if (index-first)%unit != 0 {
+		if ! isTick(len(values), unit, index) {
 			continue
 		}
 		if isVertical {
@@ -674,15 +685,13 @@ func (p *Painter) MultiText(opt MultiTextOption) *Painter {
 	}
 	count := len(opt.TextList)
 	positionCenter := true
-	showIndex := opt.Unit / 2
+	tickLimit := true
 	if containsString([]string{
 		PositionLeft,
 		PositionTop,
 	}, opt.Position) {
 		positionCenter = false
 		count--
-		// 非居中
-		showIndex = 0
 	}
 	width := p.Width()
 	height := p.Height()
@@ -690,6 +699,7 @@ func (p *Painter) MultiText(opt MultiTextOption) *Painter {
 	isVertical := opt.Orient == OrientVertical
 	if isVertical {
 		values = autoDivide(height, count)
+		tickLimit = false
 	} else {
 		values = autoDivide(width, count)
 	}
@@ -699,7 +709,7 @@ func (p *Painter) MultiText(opt MultiTextOption) *Painter {
 		if index < opt.First {
 			continue
 		}
-		if opt.Unit != 0 && (index-opt.First)%opt.Unit != showIndex {
+		if opt.Unit != 0 && tickLimit && ! isTick(len(opt.TextList)-opt.First, opt.Unit, index-opt.First) {
 			continue
 		}
 		if isTextRotation {
@@ -724,7 +734,11 @@ func (p *Painter) MultiText(opt MultiTextOption) *Painter {
 				x = 0
 			}
 		} else {
-			x = start - box.Width()>>1
+			if index == len(opt.TextList) - 1 {
+				x = start - box.Width()
+			} else {
+				x = start - box.Width()>>1
+			}
 		}
 		x += offset.Left
 		y += offset.Top
@@ -749,7 +763,6 @@ func (p *Painter) Grid(opt GridOption) *Painter {
 			x1 := 0
 			y1 := 0
 			if isVertical {
-
 				x0 = v
 				x1 = v
 				y1 = height