Fix for enabling monotone cubicInterpolationMode when tension is 0

src/elements/element.line.js

@@ -27,7 +27,7 @@ function getLineMethod(options) {
     return _steppedLineTo;
   }
 
-  if (options.tension) {
+  if (options.tension || options.cubicInterpolationMode === 'monotone') {
     return _bezierCurveTo;
   }
 
@@ -182,7 +182,7 @@ function fastPathSegment(ctx, line, segment, params) {
 function _getSegmentMethod(line) {
   const opts = line.options;
   const borderDash = opts.borderDash && opts.borderDash.length;
-  const useFastPath = !line._decimated && !line._loop && !opts.tension && !opts.stepped && !borderDash;
+  const useFastPath = !line._decimated && !line._loop && !opts.tension && opts.cubicInterpolationMode !== 'monotone' && !opts.stepped && !borderDash;
   return useFastPath ? fastPathSegment : pathSegment;
 }
 
@@ -194,7 +194,7 @@ function _getInterpolationMethod(options) {
     return _steppedInterpolation;
   }
 
-  if (options.tension) {
+  if (options.tension || options.cubicInterpolationMode === 'monotone') {
     return _bezierInterpolation;
   }
 
@@ -260,7 +260,7 @@ export default class LineElement extends Element {
   updateControlPoints(chartArea) {
     const me = this;
     const options = me.options;
-    if (options.tension && !options.stepped && !me._pointsUpdated) {
+    if ((options.tension || options.cubicInterpolationMode === 'monotone') && !options.stepped && !me._pointsUpdated) {
       const loop = options.spanGaps ? me._loop : me._fullLoop;
       _updateBezierControlPoints(me._points, options, chartArea, loop);
       me._pointsUpdated = true;

test/fixtures/element.line/cubicInterpolationMode/monotone.js

@@ -0,0 +1,27 @@
+module.exports = {
+  config: {
+    type: 'line',
+    data: {
+      datasets: [
+        {
+          data: [{x: 1, y: 10}, {x: 5, y: 0}, {x: 15, y: -10}, {x: 19, y: -5}],
+          borderColor: 'red',
+          fill: false,
+          cubicInterpolationMode: 'monotone'
+        }
+      ]
+    },
+    options: {
+      scales: {
+        x: {type: 'linear', display: false, min: 0, max: 20},
+        y: {display: false, min: -15, max: 15}
+      }
+    }
+  },
+  options: {
+    canvas: {
+      height: 256,
+      width: 512
+    }
+  }
+};