do not constrain legend (x,y) when margin.autoexpand:false

- when one sets autoexpand to false, fullLayout.margin are
  honoured no matter how big the legend gets. In this case, (I'd argue)
  the legend should NOT move from its provided (or default) (x,y)
  position to make it fit on the graph.
- if some users want the legend to auto-expand the margin while keeping
  the provided/default (x,y), we could eventually add a `legend.fitinside`
  boolean attribute

Author: etpinard

src/components/legend/draw.js

@@ -101,8 +101,10 @@ module.exports = function draw(gd) {
             var lx = gs.l + gs.w * opts.x - FROM_TL[getXanchor(opts)] * opts._width;
             var ly = gs.t + gs.h * (1 - opts.y) - FROM_TL[getYanchor(opts)] * opts._effHeight;
 
-            lx = Lib.constrain(lx, 0, fullLayout.width - opts._width);
-            ly = Lib.constrain(ly, 0, fullLayout.height - opts._effHeight);
+            if(fullLayout.margin.autoexpand) {
+                lx = Lib.constrain(lx, 0, fullLayout.width - opts._width);
+                ly = Lib.constrain(ly, 0, fullLayout.height - opts._effHeight);
+            }
 
             // Set size and position of all the elements that make up a legend:
             // legend, background and border, scroll box and scroll bar

test/image/baselines/legend_margin-autoexpand-false.png

@@ -0,0 +1,19 @@
+{
+  "data": [
+    {
+      "labels": [
+        "hello",
+        "this is a very long string and bad things are likely to happen",
+        "short string",
+        "moderately long string"
+      ],
+      "values": [4, 4, 4, 4],
+      "type": "pie"
+    }
+  ],
+  "layout": {
+    "width": 400,
+    "height": 400,
+    "margin": {"autoexpand": false}
+  }
+}