Set x appropriately for series but not point selection (#779)

auto_tests/tests/plugins_legend.js

@@ -113,6 +113,18 @@ it('should use a legendFormatter', function() {
   assert.equal(calls[2].series[0].y, undefined);
 });
 
+it('should work with highlight series', () => {
+  var calls = [];
+  var g = new Dygraph(graph, 'X,y1,y2\n1,2,3\n', {
+    highlightSeriesOpts: {
+      strokeWidth: 3,
+    }
+  });
+
+  g.setSelection(false, 'y2');
+  assert.equal(Util.getLegend(graph), '');
+});
+
 it('should include point drawn where canvas-y is 0', function () {
     var graph = document.getElementById("graph");
     var calls = []

src/dygraph.js

@@ -1710,8 +1710,8 @@ Dygraph.prototype.animateSelection_ = function(direction) {
 Dygraph.prototype.updateSelection_ = function(opt_animFraction) {
   /*var defaultPrevented = */
   this.cascadeEvents_('select', {
-    selectedRow: this.lastRow_,
-    selectedX: this.lastx_,
+    selectedRow: this.lastRow_ === -1 ? undefined : this.lastRow_,
+    selectedX: this.lastx_ === -1 ? undefined : this.lastx_,
     selectedPoints: this.selPoints_
   });
   // TODO(danvk): use defaultPrevented here?
@@ -1787,6 +1787,10 @@ Dygraph.prototype.updateSelection_ = function(opt_animFraction) {
  * Manually set the selected points and display information about them in the
  * legend. The selection can be cleared using clearSelection() and queried
  * using getSelection().
+ *
+ * To set a selected series but not a selected point, call setSelection with
+ * row=false and the selected series name.
+ *
  * @param {number} row Row number that should be highlighted (i.e. appear with
  * hover dots on the chart).
  * @param {seriesName} optional series name to highlight that series with the