step 1,2,3 instead of 1,1,2,3. some chore and a comment

src/core/core.animations.js

@@ -93,21 +93,24 @@ module.exports = {
 	 */
 	advance: function() {
 		var animations = this.animations;
-		var animation, chart, nextStep;
+		var animation, chart, steps, nextStep;
 		var i = 0;
 
+		// 1 animation per chart, so we are looping charts here
 		while (i < animations.length) {
 			animation = animations[i];
 			chart = animation.chart;
+			steps = animation.numSteps;
 
-			nextStep = (animation.currentStep || 0) + 1;
-			animation.currentStep = Math.floor((Date.now() - animation.startTime) / animation.duration * animation.numSteps);
-			animation.currentStep = Math.min(Math.max(nextStep, animation.currentStep), animation.numSteps);
+			// Make sure that currentStep starts at 1
+			// https://github.com/chartjs/Chart.js/issues/6104
+			nextStep = Math.floor((Date.now() - animation.startTime) / animation.duration * steps) + 1;
+			animation.currentStep = Math.min(nextStep, steps);
 
 			helpers.callback(animation.render, [chart, animation], chart);
 			helpers.callback(animation.onAnimationProgress, [animation], chart);
 
-			if (animation.currentStep >= animation.numSteps) {
+			if (animation.currentStep >= steps) {
 				helpers.callback(animation.onAnimationComplete, [animation], chart);
 				chart.animating = false;
 				animations.splice(i, 1);