fix label's maxWidth calculation

jquery.flot.js

@@ -1347,7 +1347,7 @@ Licensed under the MIT license.
                 ticks = axis.ticks || [],
                 labelWidth = opts.labelWidth || 0,
                 labelHeight = opts.labelHeight || 0,
-                maxWidth = labelWidth || axis.direction == "x" ? Math.floor(surface.width / (ticks.length || 1)) : null,
+                maxWidth = labelWidth || (axis.direction == "x" ? Math.floor(surface.width / (ticks.length || 1)) : null),
                 legacyStyles = axis.direction + "Axis " + axis.direction + axis.n + "Axis",
                 layer = "flot-" + axis.direction + "-axis flot-" + axis.direction + axis.n + "-axis " + legacyStyles,
                 font = opts.font || "flot-tick-label tickLabel";