don't (uselessly) add data-index to ann text+bg & arrow groups

- one `data-index` attr on the top-level ann group
  should be enough.

Author: etpinard

src/components/annotations/draw.js

@@ -101,8 +101,7 @@ function drawOne(gd, index) {
 
     // another group for text+background so that they can rotate together
     var annTextGroup = annGroup.append('g')
-        .classed('annotation-text-g', true)
-        .attr('data-index', String(index));
+        .classed('annotation-text-g', true);
 
     var annTextGroupInner = annTextGroup.append('g')
         .style('pointer-events', options.captureevents ? 'all' : null)
@@ -474,8 +473,7 @@ function drawOne(gd, index) {
 
             var arrowGroup = annGroup.append('g')
                 .style({opacity: Color.opacity(arrowColor)})
-                .classed('annotation-arrow-g', true)
-                .attr('data-index', String(index));
+                .classed('annotation-arrow-g', true);
 
             var arrow = arrowGroup.append('path')
                 .attr('d', 'M' + tailX + ',' + tailY + 'L' + headX + ',' + headY)
@@ -498,7 +496,6 @@ function drawOne(gd, index) {
                     .classed('annotation', true)
                     .classed('anndrag', true)
                     .attr({
-                        'data-index': String(index),
                         d: 'M3,3H-3V-3H3ZM0,0L' + (tailX - arrowDragHeadX) + ',' + (tailY - arrowDragHeadY),
                         transform: 'translate(' + arrowDragHeadX + ',' + arrowDragHeadY + ')'
                     })