Fix wrong default usage in StackedBarChart

Using width instead of height.

Signed-off-by: Vinicius Tinti <vinicius.tinti@almg.gov.br>

stacked_bar_chart.go

@@ -82,7 +82,7 @@ func (sbc StackedBarChart) GetWidth() int {
 // GetHeight returns the chart height or the default value.
 func (sbc StackedBarChart) GetHeight() int {
 	if sbc.Height == 0 {
-		return DefaultChartWidth
+		return DefaultChartHeight
 	}
 	return sbc.Height
 }