Show only integer ticks in stepAxis (#1396)

A step is a discrete entity, not a contiguous one. Plottable, without a
custom tick generator, tries to generate predefiend number of ticks
which can result in decimal ticks when domain is small enough.

Instead, we now show only ticks on integer values.

Author: stephanwlee

tensorboard/components/vz_chart_helpers/vz-chart-helpers.ts

@@ -196,6 +196,7 @@ export let stepFormatter =
     Plottable.Formatters.siSuffix(STEP_FORMATTER_PRECISION);
 export function stepX(): XComponents {
   let scale = new Plottable.Scales.Linear();
+  scale.tickGenerator(Plottable.Scales.TickGenerators.integerTickGenerator());
   let axis = new Plottable.Axes.Numeric(scale, 'bottom');
   axis.formatter(stepFormatter);
   return {