Log-scale layout of heatmap bricks for log-scale axes (fixes #5991)

draftlogs/6891_change.md

@@ -0,0 +1 @@
+- Improve rendering of heatmap bricks for log-scale axes [[#5991](https://github.com/plotly/plotly.js/issues/5991)], with thanks to @andrew-matteson for the contribution!

src/traces/heatmap/make_bound_array.js

@@ -24,11 +24,25 @@ module.exports = function makeBoundArray(trace, arrayIn, v0In, dvIn, numbricks,
             // contour plots only want the centers
             if(isContour || isGL2D) arrayOut = Array.from(arrayIn).slice(0, numbricks);
             else if(numbricks === 1) {
-                arrayOut = [arrayIn[0] - 0.5, arrayIn[0] + 0.5];
+                if(ax.type === 'log') {
+                    arrayOut = [0.5 * arrayIn[0], 2 * arrayIn[0]];
+                } else {
+                    arrayOut = [arrayIn[0] - 0.5, arrayIn[0] + 0.5];
+                }
+            } else if(ax.type === 'log') {
+                arrayOut = [Math.pow(arrayIn[0], 1.5) / Math.pow(arrayIn[1], 0.5)];
+
+                for(i = 1; i < len; i++) {
+                    // Geomean
+                    arrayOut.push(Math.sqrt(arrayIn[i - 1] * arrayIn[i]));
+                }
+
+                arrayOut.push(Math.pow(arrayIn[len - 1], 1.5) / Math.pow(arrayIn[len - 2], 0.5));
             } else {
                 arrayOut = [1.5 * arrayIn[0] - 0.5 * arrayIn[1]];
 
                 for(i = 1; i < len; i++) {
+                    // Arithmetic mean
                     arrayOut.push((arrayIn[i - 1] + arrayIn[i]) * 0.5);
                 }
 
@@ -37,11 +51,21 @@ module.exports = function makeBoundArray(trace, arrayIn, v0In, dvIn, numbricks,
 
             if(len < numbricks) {
                 var lastPt = arrayOut[arrayOut.length - 1];
-                var delta = lastPt - arrayOut[arrayOut.length - 2];
+                var delta; // either multiplicative delta (log axis type) or arithmetic delta (all other axis types)
+                if(ax.type === 'log') {
+                    delta = lastPt / arrayOut[arrayOut.length - 2];
+
+                    for(i = len; i < numbricks; i++) {
+                        lastPt *= delta;
+                        arrayOut.push(lastPt);
+                    }
+                } else {
+                    delta = lastPt - arrayOut[arrayOut.length - 2];
 
-                for(i = len; i < numbricks; i++) {
-                    lastPt += delta;
-                    arrayOut.push(lastPt);
+                    for(i = len; i < numbricks; i++) {
+                        lastPt += delta;
+                        arrayOut.push(lastPt);
+                    }
                 }
             }
         } else {

test/image/mocks/zz-heatmap-log-scale.json

@@ -0,0 +1,47 @@
+{
+    "data": [
+        {
+            "type": "heatmap",
+            "x": [
+                0.0001,
+                0.001,
+                0.01
+            ],
+            "y": [
+                0.0001,
+                0.001,
+                0.01
+            ],
+            "z": [
+                [
+                    1,
+                    2,
+                    3
+                ],
+                [
+                    3,
+                    2,
+                    1
+                ],
+                [
+                    7,
+                    2,
+                    7
+                ]
+            ]
+        }
+    ],
+    "layout": {
+        "title": {"text": "log scale heatmap"},
+        "xaxis": {
+            "type": "log"
+        },
+        "yaxis": {
+            "type": "log"
+        },
+        "height": 598,
+        "width": 1080,
+        "autosize": true,
+        "showlegend": false
+    }
+}