arrow_paths noRotate option

instead of hard-coding these in drawArrowHead

Author: alexcjohnson

src/components/annotations/arrow_paths.js

@@ -12,10 +12,15 @@
  * All paths are tuned for maximum scalability of the arrowhead,
  * ie throughout arrowwidth=0.3..3 the head is joined smoothly
  * to the line, with the line coming from the left and ending at (0, 0).
+ *
  * `backoff` is the distance to move the arrowhead and the end of the line,
  * in order that the arrowhead points to the desired place, either at
  * the tip of the arrow or (in the case of circle or square)
  * the center of the symbol.
+ *
+ * `noRotate`, if truthy, says that this arrowhead should not rotate with the
+ * arrow. That's the case for squares, which should always be straight, and
+ * circles, for which it's irrelevant.
  */
 
 module.exports = [
@@ -52,11 +57,13 @@ module.exports = [
     // circle
     {
         path: 'M2,0A2,2 0 1,1 0,-2A2,2 0 0,1 2,0Z',
-        backoff: 0
+        backoff: 0,
+        noRotate: true
     },
     // square
     {
         path: 'M2,2V-2H-2V2Z',
-        backoff: 0
+        backoff: 0,
+        noRotate: true
     }
 ];

src/components/annotations/draw_arrow_head.js

@@ -113,15 +113,15 @@ module.exports = function drawArrowHead(el3, ends, options) {
 
     function drawhead(p, rot) {
         if(!headStyle.path) return;
-        if(options.arrowhead > 5) rot = 0; // don't rotate square or circle
+        if(headStyle.noRotate) rot = 0;
 
         d3.select(el.parentNode).append('path')
             .attr({
                 'class': el3.attr('class'),
                 d: headStyle.path,
                 transform:
                     'translate(' + p.x + ',' + p.y + ')' +
-                    'rotate(' + (rot * 180 / Math.PI) + ')' +
+                    (rot ? 'rotate(' + (rot * 180 / Math.PI) + ')' : '') +
                     'scale(' + scale + ')'
             })
             .style({