Check whether values are constant before smooth (#1698)

Due to IEEE 754 floating-point precision, multiplying floating smoothing
factor into a value can cause discrepancy in otherwise mathematically
constant value.

Please see #786 for examples of weird spikes/messed up y-scale.

Partially addresses #786.

Author: stephanwlee

tensorboard/components/vz_line_chart/vz-line-chart.ts

@@ -1015,9 +1015,13 @@ class LineChart {
     // frequency components of the time-series.
     let last = data.length > 0 ? 0 : NaN;
     let numAccum = 0;
+
+    const yValues = data.map((d, i) => this.yValueAccessor(d, i, dataset));
+    // See #786.
+    const isConstant = yValues.every((v) => v == yValues[0]);
     data.forEach((d, i) => {
-      let nextVal = this.yValueAccessor(d, i, dataset);
-      if (!Number.isFinite(nextVal)) {
+      const nextVal = yValues[i];
+      if (isConstant || !Number.isFinite(nextVal)) {
         d.smoothed = nextVal;
       } else {
         last = last * smoothingWeight + (1 - smoothingWeight) * nextVal;

tensorboard/components/vz_line_chart2/line-chart.ts

@@ -707,9 +707,13 @@ export class LineChart {
     // frequency components of the time-series.
     let last = data.length > 0 ? 0 : NaN;
     let numAccum = 0;
+
+    const yValues = data.map((d, i) => this.yValueAccessor(d, i, dataset));
+    // See #786.
+    const isConstant = yValues.every((v) => v == yValues[0]);
     data.forEach((d, i) => {
-      let nextVal = this.yValueAccessor(d, i, dataset);
-      if (!Number.isFinite(nextVal)) {
+      const nextVal = yValues[i];
+      if (isConstant || !Number.isFinite(nextVal)) {
         d.smoothed = nextVal;
       } else {
         last = last * smoothingWeight + (1 - smoothingWeight) * nextVal;