Use static offsets for panning rounded time scale

karma.conf.js

@@ -62,6 +62,7 @@ module.exports = function(karma) {
       {pattern: 'node_modules/chart.js/dist/chart.js'},
       {pattern: 'node_modules/hammer-simulator/index.js'},
       {pattern: 'node_modules/hammerjs/hammer.js'},
+      {pattern: 'node_modules/chartjs-adapter-date-fns/dist/chartjs-adapter-date-fns.bundle.js'},
       {pattern: 'test/index.js'},
       {pattern: 'src/index.js'},
       {pattern: 'test/specs/**/*.js'}

src/core.js

@@ -77,8 +77,17 @@ export function resetZoom(chart) {
 }
 
 function panScale(scale, delta, panOptions, limits) {
+  const {panDelta} = getState(scale.chart);
+  // Add possible cumulative delta from previous pan attempts where scale did not change
+  delta += panDelta[scale.id] || 0;
   const fn = panFunctions[scale.type] || panFunctions.default;
-  call(fn, [scale, delta, panOptions, limits]);
+  if (call(fn, [scale, delta, panOptions, limits])) {
+    // The scale changed, reset cumulative delta
+    panDelta[scale.id] = 0;
+  } else {
+    // The scale did not change, store cumulative delta
+    panDelta[scale.id] = delta;
+  }
 }
 
 export function doPan(chart, pan, enabledScales) {

src/hammer.js

@@ -80,7 +80,6 @@ function endPinch(chart, state, e) {
   }
 }
 
-
 function handlePan(chart, state, e) {
   const delta = state.delta;
   if (delta !== null) {

src/scale.types.js

@@ -39,12 +39,14 @@ function updateRange(scale, {min, max}, limits, zoom = false) {
   }
   scaleOpts.min = min;
   scaleOpts.max = max;
+  // return true if the scale range is changed
+  return scale.parse(min) !== scale.min || scale.parse(max) !== scale.max;
 }
 
 function zoomNumericalScale(scale, zoom, center, limits) {
   const delta = zoomDelta(scale, zoom, center);
   const newRange = {min: scale.min + delta.min, max: scale.max - delta.max};
-  updateRange(scale, newRange, limits, true);
+  return updateRange(scale, newRange, limits, true);
 }
 
 const integerChange = (v) => v === 0 || isNaN(v) ? 0 : v < 0 ? Math.min(Math.round(v), -1) : Math.max(Math.round(v), 1);
@@ -61,7 +63,7 @@ function zoomCategoryScale(scale, zoom, center, limits) {
   }
   const delta = zoomDelta(scale, zoom, center);
   const newRange = {min: scale.min + integerChange(delta.min), max: scale.max - integerChange(delta.max)};
-  updateRange(scale, newRange, limits, true);
+  return updateRange(scale, newRange, limits, true);
 }
 
 const categoryDelta = new WeakMap();
@@ -80,14 +82,27 @@ function panCategoryScale(scale, delta, panOptions, limits) {
 
   categoryDelta.set(scale, minIndex !== scaleMin ? 0 : cumDelta);
 
-  updateRange(scale, {min: minIndex, max: maxIndex}, limits);
+  return updateRange(scale, {min: minIndex, max: maxIndex}, limits);
 }
 
+const OFFSETS = {
+  second: 500, // 500 ms
+  minute: 30 * 1000, // 30 s
+  hour: 30 * 60 * 1000, // 30 m
+  day: 12 * 60 * 60 * 1000, // 12 h
+  week: 3.5 * 24 * 60 * 60 * 1000, // 3.5 d
+  month: 15 * 24 * 60 * 60 * 1000, // 15 d
+  quarter: 60 * 24 * 60 * 60 * 1000, // 60 d
+  year: 182 * 24 * 60 * 60 * 1000 // 182 d
+};
+
 function panNumericalScale(scale, delta, panOptions, limits) {
-  const {min: prevStart, max: prevEnd} = scale;
-  const newMin = scale.getValueForPixel(scale.getPixelForValue(prevStart) - delta);
-  const newMax = scale.getValueForPixel(scale.getPixelForValue(prevEnd) - delta);
-  updateRange(scale, {min: newMin, max: newMax}, limits);
+  const {min: prevStart, max: prevEnd, options} = scale;
+  const round = options.time && options.time.round;
+  const offset = OFFSETS[round] || 0;
+  const newMin = scale.getValueForPixel(scale.getPixelForValue(prevStart + offset) - delta);
+  const newMax = scale.getValueForPixel(scale.getPixelForValue(prevEnd + offset) - delta);
+  return updateRange(scale, {min: newMin, max: newMax}, limits);
 }
 
 export const zoomFunctions = {

src/state.js

@@ -6,6 +6,7 @@ export function getState(chart) {
     state = {
       originalScaleLimits: {},
       handlers: {},
+      panDelta: {}
     };
     chartStates.set(chart, state);
   }

test/fixtures/pan/time-day-left.js

@@ -0,0 +1,66 @@
+const canvas = document.createElement('canvas');
+canvas.width = 512;
+canvas.height = 512;
+const ctx = canvas.getContext('2d');
+
+module.exports = {
+  config: {
+    type: 'bar',
+    data: {
+      labels: [new Date(2020, 0, 2), new Date(2020, 0, 3), new Date(2020, 0, 4), new Date(2020, 0, 5)],
+      datasets: [
+        {
+          backgroundColor: ['red', 'green', 'blue', 'orange'],
+          data: [1, 2, 3, 4]
+        }
+      ]
+    },
+    options: {
+      animation: false,
+      events: [],
+      scales: {
+        y: {
+          display: false,
+          max: 5
+        },
+        x: {
+          type: 'time',
+          min: new Date(2020, 0, 3),
+          max: new Date(2020, 0, 5),
+          time: {
+            unit: 'day',
+            round: 'day',
+          }
+        }
+      },
+      plugins: {
+        legend: false,
+        zoom: {
+          pan: {
+            enabled: true,
+            mode: 'x',
+          }
+        }
+      }
+    }
+  },
+  options: {
+    spriteText: true,
+    run(chart) {
+      const steps = 4;
+      const n = Math.sqrt(steps);
+      const side = 512 / n;
+      for (let i = 0; i < steps; i++) {
+        const col = i % n;
+        const row = Math.floor(i / n);
+        if (i > 0) {
+          chart.pan({x: 150});
+          chart.update();
+        }
+        ctx.drawImage(chart.canvas, col * side, row * side, side, side);
+      }
+      Chart.helpers.clearCanvas(chart.canvas);
+      chart.ctx.drawImage(canvas, 0, 0);
+    }
+  }
+};

test/fixtures/pan/time-day-right.js

@@ -0,0 +1,66 @@
+const canvas = document.createElement('canvas');
+canvas.width = 512;
+canvas.height = 512;
+const ctx = canvas.getContext('2d');
+
+module.exports = {
+  config: {
+    type: 'bar',
+    data: {
+      labels: [new Date(2020, 0, 2), new Date(2020, 0, 3), new Date(2020, 0, 4), new Date(2020, 0, 5)],
+      datasets: [
+        {
+          backgroundColor: ['red', 'green', 'blue', 'orange'],
+          data: [1, 2, 3, 4]
+        }
+      ]
+    },
+    options: {
+      animation: false,
+      events: [],
+      scales: {
+        y: {
+          display: false,
+          max: 5
+        },
+        x: {
+          type: 'time',
+          min: new Date(2020, 0, 1),
+          max: new Date(2020, 0, 3),
+          time: {
+            unit: 'day',
+            round: 'day',
+          }
+        }
+      },
+      plugins: {
+        legend: false,
+        zoom: {
+          pan: {
+            enabled: true,
+            mode: 'x',
+          }
+        }
+      }
+    }
+  },
+  options: {
+    spriteText: true,
+    run(chart) {
+      const steps = 4;
+      const n = Math.sqrt(steps);
+      const side = 512 / n;
+      for (let i = 0; i < steps; i++) {
+        const col = i % n;
+        const row = Math.floor(i / n);
+        if (i > 0) {
+          chart.pan({x: -150});
+          chart.update();
+        }
+        ctx.drawImage(chart.canvas, col * side, row * side, side, side);
+      }
+      Chart.helpers.clearCanvas(chart.canvas);
+      chart.ctx.drawImage(canvas, 0, 0);
+    }
+  }
+};