should be center - there is no middle xanchor for colorbar

Author: archmoj

src/components/colorbar/draw.js

@@ -208,7 +208,7 @@ function drawColorBar(g, opts, gd) {
     // then fix at the end (since we don't know the width yet)
     var xLeft = Math.round(opts.x * gs.w + opts.xpad);
     // for dragging... this is getting a little muddled...
-    var xLeftFrac = opts.x - thickFrac * ({middle: 0.5, right: 1}[opts.xanchor] || 0);
+    var xLeftFrac = opts.x - thickFrac * ({center: 0.5, right: 1}[opts.xanchor] || 0);
 
     // y positioning we can do correctly from the start
     var yBottomFrac = opts.y + lenFrac * (({top: -0.5, bottom: 0.5}[opts.yanchor] || 0) - 0.5);