fix carpet ax title position for decreasing a/b coords

Author: etpinard

src/traces/carpet/plot.js

@@ -173,17 +173,20 @@ function drawAxisLabels(gd, xaxis, yaxis, trace, t, layer, labels, labelClass) {
 function drawAxisTitles(gd, layer, trace, t, xa, ya, labelOrientationA, labelOrientationB) {
     var a, b, xy, dxy;
 
-    a = 0.5 * (trace.a[0] + trace.a[trace.a.length - 1]);
-    b = trace.b[0];
+    var _a = trace.a.slice().sort(Lib.sorterAsc);
+    var _b = trace.b.slice().sort(Lib.sorterAsc);
+
+    a = 0.5 * (_a[0] + _a[_a.length - 1]);
+    b = _b[0];
     xy = trace.ab2xy(a, b, true);
     dxy = trace.dxyda_rough(a, b);
     if(labelOrientationA.angle === undefined) {
         Lib.extendFlat(labelOrientationA, orientText(trace, xa, ya, xy, trace.dxydb_rough(a, b)));
     }
     drawAxisTitle(gd, layer, trace, t, xy, dxy, trace.aaxis, xa, ya, labelOrientationA, 'a-title');
 
-    a = trace.a[0];
-    b = 0.5 * (trace.b[0] + trace.b[trace.b.length - 1]);
+    a = _a[0];
+    b = 0.5 * (_b[0] + _b[_b.length - 1]);
     xy = trace.ab2xy(a, b, true);
     dxy = trace.dxydb_rough(a, b);
     if(labelOrientationB.angle === undefined) {

test/image/baselines/carpet_ordering-labeling.png

@@ -0,0 +1,47 @@
+{
+  "data": [
+    {
+      "type": "carpet",
+      "a": [3, 2, 1, 3, 2, 1, 3, 2, 1],
+      "aaxis": {
+        "title": { "text": "a" }
+      },
+      "b": [3, 3, 3, 2, 2, 2, 1, 1, 1],
+      "baxis": {
+        "title": { "text": "b" }
+      },
+      "y": [10, 7, 4, 7, 5, 3, 4, 3, 2]
+    },
+    {
+      "type": "carpet",
+      "a": [1, 2, 3, 1, 2, 3, 1, 2, 3],
+      "aaxis": {
+        "title": { "text": "a" }
+      },
+      "b": [1, 1, 1, 2, 2, 2, 3, 3, 3],
+      "baxis": {
+        "title": { "text": "b" }
+      },
+      "y": [2, 3, 4, 3, 5, 7, 4, 7, 10],
+      "yaxis": "y2"
+    }
+  ],
+  "layout": {
+    "grid": {
+      "rows": 2,
+      "columns": 1
+    },
+    "yaxis": {
+      "title": {
+        "text": "decreaing a/b ordering"
+      }
+    },
+    "yaxis2": {
+      "title": {
+        "text": "increaing a/b ordering"
+      }
+    },
+    "width": 600,
+    "height": 800
+  }
+}