Added a 'zero' option for lines and bars.

Area and bar plots normally use a minimum of zero, since their purpose
is to show size, and using an auto-scaled minimum distorts the plot's
meaning.  But this behavior is undesirable in cases where the plot type
is used in more of a decorative sense.

The zero option provides a way to control this behavior.  It defauls to
true for bars and filled lines.

jquery.flot.js

@@ -106,6 +106,8 @@ Licensed under the MIT license.
                         fill: false,
                         fillColor: null,
                         steps: false
+                        // Omit 'zero', so we can later default its value to
+                        // match that of the 'fill' option.
                     },
                     bars: {
                         show: false,
@@ -114,7 +116,8 @@ Licensed under the MIT license.
                         fill: true,
                         fillColor: null,
                         align: "left", // "left", "right", or "center"
-                        horizontal: false
+                        horizontal: false,
+                        zero: true
                     },
                     shadowSize: 3,
                     highlightColor: null
@@ -498,6 +501,13 @@ Licensed under the MIT license.
                         s.lines.show = true;
                 }
 
+				// 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) {
+					s.lines.zero = !!s.lines.fill;
+				}
+
                 // setup axes
                 s.xaxis = getOrCreateAxis(xaxes, axisNumber(s, "x"));
                 s.yaxis = getOrCreateAxis(yaxes, axisNumber(s, "y"));