Handle the beginAtZero option for a linear scale. Added a helper for Math.sign

src/Chart.Core.js

@@ -327,6 +327,17 @@
         min = helpers.min = function(array) {
             return Math.min.apply(Math, array);
         },
+		sign = helpers.sign = function(x) {
+			if (Math.sign) {
+				return Math.sign(x);
+			} else {
+				x = +x; // convert to a number
+				if (x === 0 || isNaN(x)) {
+					return x;
+				}
+				return x > 0 ? 1 : -1;
+			}
+		},
         cap = helpers.cap = function(valueToCap, maxValue, minValue) {
             if (isNumber(maxValue)) {
                 if (valueToCap > maxValue) {

src/Chart.Scale.js

@@ -348,7 +348,16 @@
 				// do nothing since that would make the chart weird. If the user really wants a weird chart
 				// axis, they can manually override it
 				if (this.options.beginAtZero) {
-					this.min = Math.min(this.min, 0);
+					var minSign = helpers.sign(this.min);
+					var maxSign = helpers.sign(this.max);
+					
+					if (minSign < 0 && maxSign < 0) {
+						// move the top up to 0
+						this.max = 0;
+					} else if (minSign > 0 && maxSign > 0) {
+						// move the botttom down to 0
+						this.min = 0;
+					}
 				}
 				
 				var niceRange = helpers.niceNum(this.max - this.min, false);