Fix scale when data is all small numbers (#5723)

* Add test for correct handling of small numbers

* Calculate tick precision for arbitrarily small numbers

* Use scientific notation for very small tick numbers

* Calculate significant digits for exponential tick values

src/core/core.ticks.js

@@ -46,9 +46,15 @@ module.exports = {
 			var tickString = '';
 
 			if (tickValue !== 0) {
-				var numDecimal = -1 * Math.floor(logDelta);
-				numDecimal = Math.max(Math.min(numDecimal, 20), 0); // toFixed has a max of 20 decimal places
-				tickString = tickValue.toFixed(numDecimal);
+				var maxTick = Math.max(Math.abs(ticks[0]), Math.abs(ticks[ticks.length - 1]));
+				if (maxTick < 1e-4) { // all ticks are small numbers; use scientific notation
+					var logTick = helpers.log10(Math.abs(tickValue));
+					tickString = tickValue.toExponential(Math.floor(logTick) - Math.floor(logDelta));
+				} else {
+					var numDecimal = -1 * Math.floor(logDelta);
+					numDecimal = Math.max(Math.min(numDecimal, 20), 0); // toFixed has a max of 20 decimal places
+					tickString = tickValue.toFixed(numDecimal);
+				}
 			} else {
 				tickString = '0'; // never show decimal places for 0
 			}

src/scales/scale.linearbase.js

@@ -54,7 +54,7 @@ function generateTicks(generationOptions, dataRange) {
 
 	precision = 1;
 	if (spacing < 1) {
-		precision = Math.pow(10, spacing.toString().length - 2);
+		precision = Math.pow(10, 1 - Math.floor(helpers.log10(spacing)));
 		niceMin = Math.round(niceMin * precision) / precision;
 		niceMax = Math.round(niceMax * precision) / precision;
 	}

test/specs/scale.linear.tests.js

@@ -212,6 +212,31 @@ describe('Linear Scale', function() {
 		expect(chart.scales.yScale0.max).toBe(90);
 	});
 
+	it('Should correctly determine the max & min data values for small numbers', function() {
+		var chart = window.acquireChart({
+			type: 'bar',
+			data: {
+				datasets: [{
+					yAxisID: 'yScale0',
+					data: [-1e-8, 3e-8, -4e-8, 6e-8]
+				}],
+				labels: ['a', 'b', 'c', 'd']
+			},
+			options: {
+				scales: {
+					yAxes: [{
+						id: 'yScale0',
+						type: 'linear'
+					}]
+				}
+			}
+		});
+
+		expect(chart.scales.yScale0).not.toEqual(undefined); // must construct
+		expect(chart.scales.yScale0.min * 1e8).toBeCloseTo(-4);
+		expect(chart.scales.yScale0.max * 1e8).toBeCloseTo(6);
+	});
+
 	it('Should correctly determine the max & min for scatter data', function() {
 		var chart = window.acquireChart({
 			type: 'line',