diff --git a/opts/x_axis.go b/opts/x_axis.go
index 2a2401345..4e79d4946 100644
--- a/opts/x_axis.go
+++ b/opts/x_axis.go
@@ -179,7 +179,7 @@ type AxisLabel struct {
 	// axis label font family
 	FontFamily string `json:"fontFamily,omitempty"`
 	// axis label font size
-	FontSize string `json:"fontSize,omitempty"`
+	FontSize int `json:"fontSize,omitempty"`
 	// Horizontal alignment of axis label
 	Align string `json:"align,omitempty"`
 	// Vertical alignment of axis label