fix overlaying axis layers

- bug noticed when both overlaying x and y axes have
  `showline: true`.
- ... but `Plotly.restyle(gd, 'visible', true, [1])` does
      not work for the added mock
- both bugs have been broken since PR
    #946
  released in 1.18.0

Author: etpinard

src/plots/cartesian/index.js

@@ -347,10 +347,10 @@ function makeSubplotLayer(plotinfo) {
         plotinfo.zerolinelayer = joinLayer(mainplotinfo.overzero, 'g', id);
 
         plotinfo.plot = joinLayer(mainplotinfo.overplot, 'g', id);
-        plotinfo.xlines = joinLayer(mainplotinfo.overlines, 'path', id);
-        plotinfo.ylines = joinLayer(mainplotinfo.overlines, 'path', id);
-        plotinfo.xaxislayer = joinLayer(mainplotinfo.overaxes, 'g', id);
-        plotinfo.yaxislayer = joinLayer(mainplotinfo.overaxes, 'g', id);
+        plotinfo.xlines = joinLayer(mainplotinfo.overlines, 'path', id + '-x');
+        plotinfo.ylines = joinLayer(mainplotinfo.overlines, 'path', id + '-y');
+        plotinfo.xaxislayer = joinLayer(mainplotinfo.overaxes, 'g', id + '-x');
+        plotinfo.yaxislayer = joinLayer(mainplotinfo.overaxes, 'g', id + '-y');
     }
 
     // common attributes for all subplots, overlays or not

test/image/baselines/overlaying-axis-lines.png

@@ -0,0 +1,16 @@
+{
+  "data": [{
+    "y": [1, 2, 3]
+  }, {
+    "x": [4, 5, 6], 
+    "y": [3, 1, 2], 
+    "xaxis": "x2",
+    "yaxis": "y2"
+  }],
+  "layout": {
+    "xaxis": {"showline": true},
+    "yaxis": {"showline": true},
+    "xaxis2": {"showline": true, "overlaying": "x", "side": "top"},
+    "yaxis2": {"showline": true, "overlaying": "y", "side": "right"}
+  }
+}