Calculate hidden minor ticks if ticklabelindex is set.

draftlogs/7735_change.md

@@ -0,0 +1 @@
+ - Change ticklabelindex to also be effective when no minor ticks are visible. [[#7735](https://github.com/plotly/plotly.js/pull/7735)]

src/plots/cartesian/axes.js

@@ -603,7 +603,7 @@ function autoShiftMonthBins(binStart, data, dtick, dataMin, calendar) {
 
 // ensure we have minor tick0 and dtick calculated
 axes.prepMinorTicks = function(mockAx, ax, opts) {
-    if(!ax.minor.dtick) {
+    if(!ax.minor?.dtick) {
         delete mockAx.dtick;
         var hasMajor = ax.dtick && isNumeric(ax._tmin);
         var mockMinorRange;
@@ -690,7 +690,7 @@ axes.prepMinorTicks = function(mockAx, ax, opts) {
         // put back the original range, to use to find the full set of minor ticks
         mockAx.range = ax.range;
     }
-    if(ax.minor._tick0Init === undefined) {
+    if(ax.minor?._tick0Init === undefined) {
         // ensure identical tick0
         mockAx.tick0 = ax.tick0;
     }
@@ -973,21 +973,23 @@ axes.calcTicks = function calcTicks(ax, opts) {
     var allTicklabelVals = [];
 
     var hasMinor = ax.minor && (ax.minor.ticks || ax.minor.showgrid);
+    // minor ticks should be calculated if they are visible or if ticklabelindex is set because then
+    // the labels are placed at minor ticks (even if invisible) instead of major ticks.
+    var calcMinor = hasMinor || ticklabelIndex;
 
     // calc major first
-    for(var major = 1; major >= (hasMinor ? 0 : 1); major--) {
+    for(var major = 1; major >= (calcMinor ? 0 : 1); major--) {
         var isMinor = !major;
 
         if(major) {
             ax._dtickInit = ax.dtick;
             ax._tick0Init = ax.tick0;
-        } else {
+        } else if (hasMinor) {
             ax.minor._dtickInit = ax.minor.dtick;
             ax.minor._tick0Init = ax.minor.tick0;
         }
 
-        var mockAx = major ? ax : Lib.extendFlat({}, ax, ax.minor);
-
+        var mockAx = major ? ax : Lib.extendFlat({}, ax, hasMinor ? ax.minor : {"minor": {}});
         if(isMinor) {
             axes.prepMinorTicks(mockAx, ax, opts);
         } else {
@@ -1074,7 +1076,7 @@ axes.calcTicks = function calcTicks(ax, opts) {
             }
         }
 
-        if(major && isPeriod) {
+        if((major || calcMinor) && isPeriod) {
             // add one item to label period before tick0
             x = axes.tickIncrement(x, dtick, !axrev, calendar);
             majorId--;
@@ -1298,9 +1300,14 @@ axes.calcTicks = function calcTicks(ax, opts) {
         } else {
             lastVisibleHead = ax._prevDateHead;
             t = setTickLabel(ax, tickVals[i]);
-            if(tickVals[i].skipLabel ||
-                ticklabelIndex && allTicklabelVals.indexOf(tickVals[i]) === -1) {
-                hideLabel(t);
+            if (ticklabelIndex) {
+                if (allTicklabelVals.indexOf(tickVals[i]) === -1) {
+                    hideLabel(t);
+                }
+            } else {
+                if (tickVals[i].skipLabel) {
+                    hideLabel(t);
+                }
             }
 
             ticksOut.push(t);

test/image/mocks/zz-ticklabelindex-hidden-minor-ticks-dtick.json

@@ -0,0 +1,28 @@
+{
+    "data": [
+      {
+        "showlegend": false,
+        "x": [
+          "2023-01-01",
+          "2024-01-01",
+          "2025-01-01",
+          "2026-01-01",
+          "2027-01-01"
+        ],
+        "type": "scatter"
+      }
+    ],
+    "layout": {
+      "width": 700,
+      "xaxis": {
+        "ticklen": 20,
+        "ticklabelindex": -1,
+        "tickformat": "%Y",
+        "ticklabelmode": "period",
+        "dtick": "M24"
+      },
+      "title": {
+        "text": "Should display 2 ticks and labels 2023 and 2025 to the left of them."
+      }
+    }
+  }

test/image/mocks/zz-ticklabelindex-hidden-minor-ticks-one-label.json

@@ -0,0 +1,27 @@
+{
+    "data": [
+      {
+        "showlegend": false,
+        "x": [
+          "2023-01-01",
+          "2024-01-01",
+          "2025-01-01",
+          "2026-01-01"
+        ],
+        "type": "scatter"
+      }
+    ],
+    "layout": {
+      "width": 700,
+      "xaxis": {
+        "ticklen": 20,
+        "ticklabelindex": -2,
+        "tickformat": "%Y",
+        "ticklabelmode": "period",
+        "dtick": "M24"
+      },
+      "title": {
+        "text": "Should display a label at 2024"
+      }
+    }
+  }

test/image/mocks/zz-ticklabelindex-hidden-minor-ticks-ticklabelstep.json

@@ -0,0 +1,29 @@
+{
+    "data": [
+      {
+        "showlegend": false,
+        "x": [
+          "2023-01-01",
+          "2024-01-01",
+          "2025-01-01",
+          "2026-01-01",
+          "2027-01-01"
+        ],
+        "type": "scatter"
+      }
+    ],
+    "layout": {
+      "width": 700,
+      "xaxis": {
+        "ticklen": 20,
+        "ticklabelindex": -1,
+        "tickformat": "%Y",
+        "ticklabelmode": "period",
+        "ticklabelstep": 2,
+        "dtick": "M12"
+      },
+      "title": {
+        "text": "Should display yearly ticks with labels at 2023 and 2025"
+      }
+    }
+  }