Restrict zero to bars and filled lines only.

This also includes a tweak to zero's default.  Previously zero only
received a value if lines were visible; now it always receives a value,
matching the behavior of other contextual options.

jquery.flot.js

@@ -504,7 +504,7 @@ Licensed under the MIT license.
                 // If nothing was provided for lines.zero, default it to match
                 // lines.fill, since areas by default should extend to zero.
 
-				if (s.lines.show && s.lines.zero == null) {
+                if (s.lines.zero == null) {
                     s.lines.zero = !!s.lines.fill;
                 }
 
@@ -556,7 +556,7 @@ Licensed under the MIT license.
                     format.push({ x: true, number: true, required: true });
                     format.push({ y: true, number: true, required: true });
 
-                    if (s.bars.show || (s.lines.show && (s.lines.fill || s.lines.zero))) {
+                    if (s.bars.show || (s.lines.show && s.lines.fill)) {
                         var autoscale = !!((s.bars.show && s.bars.zero) || (s.lines.show && s.lines.zero));
                         format.push({ y: true, number: true, required: false, defaultValue: 0, autoscale: autoscale });
                         if (s.bars.horizontal) {