Respect minRange in zoom edge cases

Addresses at least two edge cases where minRange produced unintuitive effects:

 * When zooming in and the zoom range was close to both the min and max limits, minRange adjustment would produce jitter violating the max.
 * When zooming out against a limit, range was truncated such that a zoomout from the opposite side of the chart could no-op.

However, this may have an undesirable implication for category axis zooming since zooming out against a limit can now alternate between zooming out by 1 or 2 categories whereas before it would tend to truncate to 1.

src/scale.types.js

@@ -47,24 +47,17 @@ export function updateRange(scale, {min, max}, limits, zoom = false) {
   const minLimit = getLimit(state, scale, scaleLimits, 'min', -Infinity);
   const maxLimit = getLimit(state, scale, scaleLimits, 'max', Infinity);
 
-  const cmin = Math.max(min, minLimit);
-  const cmax = Math.min(max, maxLimit);
-  const range = zoom ? Math.max(cmax - cmin, minRange) : scale.max - scale.min;
-  if (cmax - cmin !== range) {
-    if (minLimit > cmax - range) {
-      min = cmin;
-      max = cmin + range;
-    } else if (maxLimit < cmin + range) {
-      max = cmax;
-      min = cmax - range;
-    } else {
-      const offset = (range - cmax + cmin) / 2;
-      min = cmin - offset;
-      max = cmax + offset;
-    }
-  } else {
-    min = cmin;
-    max = cmax;
+  const range = zoom ? Math.max(max - min, minRange) : scale.max - scale.min;
+  const offset = (range - max + min) / 2;
+  min -= offset;
+  max += offset;
+
+  if (min < minLimit) {
+    min = minLimit;
+    max = Math.min(minLimit + range, maxLimit);
+  } else if (max > maxLimit) {
+    max = maxLimit;
+    min = Math.max(maxLimit - range, minLimit);
   }
   scaleOpts.min = min;
   scaleOpts.max = max;

test/specs/api.spec.js

@@ -114,6 +114,84 @@ describe('api', function() {
       expect(chart.scales.x.min).toBe(-100);
       expect(chart.scales.x.max).toBe(100);
     });
+
+    it('should no-op with fully constrained limits', function() {
+      const chart = window.acquireChart({
+        type: 'scatter',
+        options: {
+          scales: {
+            x: {
+              min: 0,
+              max: 100
+            },
+            y: {
+              min: 0,
+              max: 100
+            }
+          },
+          plugins: {
+            zoom: {
+              limits: {
+                x: {
+                  min: 0,
+                  max: 100,
+                  minRange: 100
+                }
+              }
+            }
+          }
+        }
+      });
+
+      chart.zoom(1.5);
+      expect(chart.scales.x.min).toBe(0);
+      expect(chart.scales.x.max).toBe(100);
+    });
+
+    it('should honor zoom changes against a limit', function() {
+      const chart = window.acquireChart({
+        type: 'scatter',
+        options: {
+          scales: {
+            x: {
+              min: 0,
+              max: 100
+            },
+            y: {
+              min: 0,
+              max: 100
+            }
+          },
+          plugins: {
+            zoom: {
+              limits: {
+                x: {
+                  min: 0,
+                  max: 100
+                }
+              }
+            }
+          }
+        }
+      });
+      chart.zoom({
+        x: 1.99,
+        focalPoint: {
+          x: chart.scales.x.getPixelForValue(0)
+        }
+      });
+      expect(chart.scales.x.min).toBe(0);
+      expect(chart.scales.x.max).toBe(1);
+
+      chart.zoom({
+        x: -98,
+        focalPoint: {
+          x: chart.scales.x.getPixelForValue(1)
+        }
+      });
+      expect(chart.scales.x.min).toBe(0);
+      expect(chart.scales.x.max).toBe(100);
+    });
   });
 
   describe('getInitialScaleBounds', function() {

test/specs/zoom.spec.js

@@ -933,7 +933,7 @@ describe('zoom', function() {
       expect(chart.scales.y.min).toBe(3);
       expect(chart.scales.y.max).toBe(5);
       chart.zoom(0.9);
-      expect(chart.scales.y.min).toBe(2);
+      expect(chart.scales.y.min).toBe(1);
       expect(chart.scales.y.max).toBe(5);
       chart.zoom(0.9);
       expect(chart.scales.y.min).toBe(1);