axis autotickangles property

when tickangles is set to "auto", this array will be searched for the first angle large enogh to prevent overlap between labels

Author: my-tien

src/plots/cartesian/axes.js

@@ -3479,6 +3479,7 @@ axes.drawLabels = function(gd, ax, opts) {
 
     var labelFns = opts.labelFns;
     var tickAngle = opts.secondary ? 0 : ax.tickangle;
+    var autoTickAngles = ax.autotickangles;
     var prevAngle = (ax._prevTickAngles || {})[cls];
 
     var tickLabels = opts.layer.selectAll('g.' + cls)
@@ -3780,12 +3781,24 @@ axes.drawLabels = function(gd, ax, opts) {
                 var pad = !isAligned ? 0 :
                     (ax.tickwidth || 0) + 2 * TEXTPAD;
 
-                var rotate90 = (tickSpacing < maxFontSize * 2.5) || ax.type === 'multicategory' || ax._name === 'realaxis';
+                const adjacent = tickSpacing;
+                const opposite = maxFontSize * 1.25;
+                const hypotenuse = Math.sqrt(Math.pow(adjacent, 2) + Math.pow(opposite, 2));
+                // sin(angle) = opposite / hypotenuse
+                const minAngle = Math.asin(opposite / hypotenuse) * (180 / Math.PI /* to degrees */);
+
+                var angle = autoTickAngles.find(angle => Math.abs(angle) >= minAngle);
+                if(angle === undefined) {
+                    // no angle larger than minAngle, just pick the largest angle
+                    angle = autoTickAngles.reduce(
+                        (currentMax, nextAngle) => Math.abs(currentMax) < Math.abs(nextAngle) ? nextAngle : currentMax
+                        , autoTickAngles[0]
+                    );
+                }
 
-                // any overlap at all - set 30 degrees or 90 degrees
                 for(i = 0; i < lbbArray.length - 1; i++) {
                     if(Lib.bBoxIntersect(lbbArray[i], lbbArray[i + 1], pad)) {
-                        autoangle = rotate90 ? 90 : 30;
+                        autoangle = angle;
                         break;
                     }
                 }

src/plots/cartesian/layout_attributes.js

@@ -809,6 +809,17 @@ module.exports = {
             'vertically.'
         ].join(' ')
     },
+    autotickangles: {
+        valType: 'data_array',
+        coerceNumber: true,
+        dflt: [30, 90],
+        editType: 'ticks',
+        description: [
+            'When `tickangle` is set to *auto*, it will be set to the first',
+            'angle in this array that is large enough to prevent label',
+            'overlap.'
+        ].join(' ')
+    },
     tickprefix: {
         valType: 'string',
         dflt: '',

src/plots/cartesian/tick_label_defaults.js

@@ -40,7 +40,10 @@ module.exports = function handleTickLabelDefaults(containerIn, containerOut, coe
             coerce('ticklabelstep');
         }
 
-        if(!options.noAng) coerce('tickangle');
+        if(!options.noAng) {
+            coerce('tickangle');
+            coerce('autotickangles');
+        }
 
         if(axType !== 'category') {
             var tickFormat = coerce('tickformat');

test/plot-schema.json

@@ -10699,6 +10699,21 @@
      },
      "role": "object"
     },
+    "autotickangles": {
+     "coerceNumber": true,
+     "description": "When `tickangle` is set to *auto*, it will be set to the first angle in this array that is large enough to prevent label overlap.",
+     "dflt": [
+      30,
+      90
+     ],
+     "editType": "ticks",
+     "valType": "data_array"
+    },
+    "autotickanglessrc": {
+     "description": "Sets the source reference on Chart Studio Cloud for `autotickangles`.",
+     "editType": "none",
+     "valType": "string"
+    },
     "autotypenumbers": {
      "description": "Using *strict* a numeric string in trace data is not converted to a number. Using *convert types* a numeric string in trace data may be treated as a number during automatic axis `type` detection. Defaults to layout.autotypenumbers.",
      "dflt": "convert types",
@@ -12039,6 +12054,21 @@
      "editType": "plot",
      "valType": "boolean"
     },
+    "autotickangles": {
+     "coerceNumber": true,
+     "description": "When `tickangle` is set to *auto*, it will be set to the first angle in this array that is large enough to prevent label overlap.",
+     "dflt": [
+      30,
+      90
+     ],
+     "editType": "ticks",
+     "valType": "data_array"
+    },
+    "autotickanglessrc": {
+     "description": "Sets the source reference on Chart Studio Cloud for `autotickangles`.",
+     "editType": "none",
+     "valType": "string"
+    },
     "autotypenumbers": {
      "description": "Using *strict* a numeric string in trace data is not converted to a number. Using *convert types* a numeric string in trace data may be treated as a number during automatic axis `type` detection. Defaults to layout.autotypenumbers.",
      "dflt": "convert types",