Fix label vertical alignment on vertical scales (chartjs#5248)

Author: MPierre9

src/core/core.scale.js

@@ -875,11 +875,15 @@ module.exports = function(Chart) {
 
 					var label = itemToDraw.label;
 					if (helpers.isArray(label)) {
-						for (var i = 0, y = 0; i < label.length; ++i) {
+						var lineCount = label.length;
+						var lineHeight = tickFont.size * 1.5;
+						var y = me.isHorizontal() ? 0 : -lineHeight * (lineCount - 1) / 2;
+
+						for (var i = 0; i < lineCount; ++i) {
 							// We just make sure the multiline element is a string here..
 							context.fillText('' + label[i], 0, y);
 							// apply same lineSpacing as calculated @ L#320
-							y += (tickFont.size * 1.5);
+							y += lineHeight;
 						}
 					} else {
 						context.fillText(label, 0, 0);