I think best approach to this issue is to change the "arc" definition and implementation. That is what I tried now.

@pi=3.1415926
@side_len=2
C0: dot invis at (0,0)
C1: dot invis at (@side_len*sin(0), @side_len*cos(0))
C2: dot invis at (@side_len*sin(2*@pi/5), @side_len*cos(2*@pi/5))
C3: dot invis at (@side_len*sin(4*@pi/5), @side_len*cos(4*@pi/5))
C4: dot invis at (@side_len*sin(6*@pi/5), @side_len*cos(6*@pi/5))
C5: dot invis at (@side_len*sin(8*@pi/5), @side_len*cos(8*@pi/5))


@rad_len = @side_len * 0.618

arc rad @rad_len from C1 to C0 color red
arc rad @rad_len from C1 to C0 cw color red
"P" "积极情绪" color purple at 1/2<C1,C0>

arc  rad @rad_len from C2 to C0 color red
arc rad @rad_len from C2 to C0 cw color red
"E" "投入"  color purple at 1/2<C2,C0>

arc  rad @rad_len from C3 to C0 color red
arc rad @rad_len from C3 to C0 cw color red
"R" "人际关系" color purple at 1/2<C3,C0>

arc  rad @rad_len from C4 to C0 color red
arc rad @rad_len from C4 to C0 cw color red
"M" "意义"color purple at 1/2<C4,C0>

arc  rad @rad_len from C5 to C0 color red
arc rad @rad_len from C5 to C0 cw color red
"A" "成就" color purple at 1/2<C5,C0>

It's great now ( which gives me a great surprise ), but can be more beautiful if we implement arc by using real circle . I tried the above code after I changed the origin code like this:

--- pikchr.y
+++ pikchr.y
@@ -994,59 +994,46 @@
 };


 /* Methods for the "arc" class */
 static void arcInit(Pik *p, PObj *pObj){
-  pObj->w = pik_value(p, "arcrad",6,0);
-  pObj->h = pObj->w;
-}
-/* Hack: Arcs are here rendered as quadratic Bezier curves rather
-** than true arcs.  Multiple reasons: (1) the legacy-PIC parameters
-** that control arcs are obscure and I could not figure out what they
-** mean based on available documentation.  (2) Arcs are rarely used,
-** and so do not seem that important.
-*/
-static PPoint arcControlPoint(int cw, PPoint f, PPoint t, PNum rScale){
-  PPoint m;
-  PNum dx, dy;
-  m.x = 0.5*(f.x+t.x);
-  m.y = 0.5*(f.y+t.y);
-  dx = t.x - f.x;
-  dy = t.y - f.y;
-  if( cw ){
-    m.x -= 0.5*rScale*dy;
-    m.y += 0.5*rScale*dx;
-  }else{
-    m.x += 0.5*rScale*dy;
-    m.y -= 0.5*rScale*dx;
-  }
-  return m;
+  if (pObj->rad == 0.0)
+    pObj->rad = pik_value(p, "arcrad",6,0);
+  PPoint f, t;
+  f = p->aTPath[0];
+  t = p->aTPath[1];
+  PNum minrad;
+  minrad = pik_dist(&f, &t) * 0.5;
+  if( pObj->rad<minrad ) pObj->rad = minrad;
+  PPoint center;
+  center.x = (f.x + t.x) * 0.5;
+  center.y = (f.y + t.y) * 0.5;
+  // make arc as large as "max circle"
+  pik_bbox_add_xy(&pObj->bbox, center.x - minrad, center.y - minrad);
+  pik_bbox_add_xy(&pObj->bbox, center.x + minrad, center.y + minrad);
+  pObj->w = pObj->h = pObj->rad;
 }
 static void arcCheck(Pik *p, PObj *pObj){
-  PPoint m;
   if( p->nTPath>2 ){
     pik_error(p, &pObj->errTok, "arc geometry error");
     return;
   }
-  m = arcControlPoint(pObj->cw, p->aTPath[0], p->aTPath[1], 0.5);
-  pik_bbox_add_xy(&pObj->bbox, m.x, m.y);
 }
 static void arcRender(Pik *p, PObj *pObj){
-  PPoint f, m, t;
+  PPoint f, t;
+  PNum r;
   if( pObj->nPath<2 ) return;
   if( pObj->sw<0.0 ) return;
   f = pObj->aPath[0];
   t = pObj->aPath[1];
-  m = arcControlPoint(pObj->cw,f,t,1.0);
-  if( pObj->larrow ){
-    pik_draw_arrowhead(p,&m,&f,pObj);
-  }
-  if( pObj->rarrow ){
-    pik_draw_arrowhead(p,&m,&t,pObj);
-  }
+  r = pObj->rad;
   pik_append_xy(p,"<path d=\"M", f.x, f.y);
-  pik_append_xy(p,"Q", m.x, m.y);
+  pik_append_dis(p,"A", r, "");
+  pik_append_dis(p, " ", r, "");
+  pik_append_num(p, " ", 0); // rotation
+  pik_append_num(p, " ", 0); // always small arc
+  pik_append_num(p, " ", pObj->cw);
   pik_append_xy(p," ", t.x, t.y);
   pik_append(p,"\" ",2);
   pik_append_style(p,pObj,0);
   pik_append(p,"\" />\n", -1);