Add option to always show selection rectangle

Previously, if the selected area was very small, the selection
rectangle would not be displayed. This commit adds an "alwaysShow"
option so that, when true, the selection rectangle will always be
displayed. When the selected area is very small, the selection
rectangle will become a line.

jquery.flot.selection.js

@@ -8,7 +8,8 @@ The plugin supports these options:
 selection: {
 	mode: null or "x" or "y" or "xy",
 	color: color,
-	shape: "round" or "miter" or "bevel"
+	shape: "round" or "miter" or "bevel",
+	alwaysShow: boolean
 }
 
 Selection support is enabled by setting the mode to one of "x", "y" or "xy".
@@ -16,7 +17,9 @@ In "x" mode, the user will only be able to specify the x range, similarly for
 "y" mode. For "xy", the selection becomes a rectangle where both ranges can be
 specified. "color" is color of the selection (if you need to change the color
 later on, you can get to it with plot.getOptions().selection.color). "shape"
-is the shape of the corners of the selection.
+is the shape of the corners of the selection. When "alwaysShow" is true,
+the selection rectangle will always be displayed (as a line), even when the
+selection is very small.
 
 When selection support is enabled, a "plotselected" event will be emitted on
 the DOM element you passed into the plot function. The event handler gets a
@@ -296,9 +299,9 @@ The plugin allso adds the following methods to the plot object:
 
         plot.hooks.drawOverlay.push(function (plot, ctx) {
             // draw selection
-            if (selection.show && selectionIsSane()) {
+            var o = plot.getOptions();
+            if (selection.show && (selectionIsSane() || o.selection.alwaysShow)) {
                 var plotOffset = plot.getPlotOffset();
-                var o = plot.getOptions();
 
                 ctx.save();
                 ctx.translate(plotOffset.left, plotOffset.top);
@@ -338,7 +341,8 @@ The plugin allso adds the following methods to the plot object:
             selection: {
                 mode: null, // one of null, "x", "y" or "xy"
                 color: "#e8cfac",
-                shape: "round" // one of "round", "miter", or "bevel"
+                shape: "round", // one of "round", "miter", or "bevel"
+                alwaysShow: false // boolean
             }
         },
         name: 'selection',