Add "selectdirection" layout attribute

Author: GlennMatthys

src/components/fx/layout_attributes.js

@@ -101,5 +101,17 @@ module.exports = {
             ].join(' ')
         },
         editType: 'none'
+    },
+    selectdirection: {
+        valType: 'enumerated',
+        role: 'info',
+        values: ['h', 'v', 'd', 'any'],
+        dflt: 'any',
+        description: [
+            'When "dragmode" is set to "select", this limits the selection of the drag to',
+            'horizontal, vertical or diagonal. "h" only allows horizontal selection,',
+            '"v" only vertical, "d" only diagonal and "any" sets no limit.'
+        ].join(' '),
+        editType: 'none'
     }
 };

src/components/fx/layout_defaults.js

@@ -17,6 +17,7 @@ module.exports = function supplyLayoutDefaults(layoutIn, layoutOut, fullData) {
     }
 
     coerce('dragmode');
+    coerce('selectdirection', 'any');
 
     var hovermodeDflt;
     if(layoutOut._has('cartesian')) {