only override grid.(x|y)side default for sploms w/o lower half

- this way, the Grid component can generate axes with set 'anchor'
  for all sploms except when `showlowerhalf: false`. Having
  set anchored axes is a performance boost (~40ms at 50 dimensions).

Author: etpinard

Diff for: src/components/grid/index.js

@@ -225,9 +225,9 @@ function sizeDefaults(layoutIn, layoutOut) {
     var dfltGapY = hasSubplotGrid ? 0.3 : 0.1;
 
     var dfltSideX, dfltSideY;
-    if(isSplomGenerated) {
-        dfltSideX = 'bottom';
-        dfltSideY = 'left';
+    if(isSplomGenerated && layoutOut._splomGridDflt) {
+        dfltSideX = layoutOut._splomGridDflt.xside;
+        dfltSideY = layoutOut._splomGridDflt.yside;
     }
 
     gridOut._domains = {

Diff for: src/plots/plots.js

@@ -382,6 +382,8 @@ plots.supplyDefaults = function(gd, opts) {
     // initialize axis and subplot hash objects for splom-generated grids
     var splomAxes = newFullLayout._splomAxes = {x: {}, y: {}};
     var splomSubplots = newFullLayout._splomSubplots = {};
+    // initialize splom grid defaults
+    newFullLayout._splomGridDflt = {};
 
     // for traces to request a default rangeslider on their x axes
     // eg set `_requestRangeslider.x2 = true` for xaxis2

Diff for: src/traces/splom/defaults.js

@@ -150,6 +150,13 @@ function handleAxisDefaults(traceIn, traceOut, layout, coerce) {
         // note that some the entries here may be undefined
         diag[i] = [xa, ya];
     }
+
+    // when lower half is omitted, override grid default
+    // to make sure axes remain on the left/bottom of the plot area
+    if(!showLower) {
+        layout._splomGridDflt.xside = 'bottom';
+        layout._splomGridDflt.yside = 'left';
+    }
 }
 
 function fillAxisIdArray(axLetter, len) {

Diff for: test/jasmine/tests/splom_test.js

@@ -100,20 +100,35 @@ describe('Test splom trace defaults:', function() {
         expect(subplots.cartesian).toEqual(['xy', 'xy2', 'x2y', 'x2y2']);
     });
 
-    it('should use special `grid.xside` and `grid.yside` defaults on splom generated grids', function() {
+    it('should use special `grid.xside` and `grid.yside` defaults on splom w/o lower half generated grids', function() {
         var gridOut;
 
+        // base case
         _supply({
             dimensions: [
                 {values: [1, 2, 3]},
                 {values: [2, 1, 2]}
             ]
         });
 
+        gridOut = gd._fullLayout.grid;
+        expect(gridOut.xside).toBe('bottom plot');
+        expect(gridOut.yside).toBe('left plot');
+
+        // w/o lower half case
+        _supply({
+            dimensions: [
+                {values: [1, 2, 3]},
+                {values: [2, 1, 2]}
+            ],
+            showlowerhalf: false
+        });
+
         gridOut = gd._fullLayout.grid;
         expect(gridOut.xside).toBe('bottom');
         expect(gridOut.yside).toBe('left');
 
+        // non-splom generated grid
         _supply({
             dimensions: [
                 {values: [1, 2, 3]},