Added BaseValue funtionality to bar chart (#94)

bar_chart.go

@@ -31,6 +31,9 @@ type BarChart struct {
 
 	BarSpacing int
 
+	UseBaseValue bool
+	BaseValue    float64
+
 	Font        *truetype.Font
 	defaultFont *truetype.Font
 
@@ -199,11 +202,20 @@ func (bc BarChart) drawBars(r Renderer, canvasBox Box, yr Range) {
 
 		by = canvasBox.Bottom - yr.Translate(bar.Value)
 
-		barBox = Box{
-			Top:    by,
-			Left:   bxl,
-			Right:  bxr,
-			Bottom: canvasBox.Bottom,
+		if bc.UseBaseValue {
+			barBox = Box{
+				Top:    by,
+				Left:   bxl,
+				Right:  bxr,
+				Bottom: canvasBox.Bottom - yr.Translate(bc.BaseValue),
+			}
+		} else {
+			barBox = Box{
+				Top:    by,
+				Left:   bxl,
+				Right:  bxr,
+				Bottom: canvasBox.Bottom,
+			}
 		}
 
 		Draw.Box(r, barBox, bar.Style.InheritFrom(bc.styleDefaultsBar(index)))