   const tooltipWidth = 110;
      const tooltipHeight = 40;
      const chartInnerWidth = this.chartWidth_ - CHART_PADDING_RIGHT -
          CHART_PADDING_LEFT;
      const barWidth = chartInnerWidth / this.opsTimingData_.length;
      const tooltipOffset = Math.round((tooltipWidth - barWidth) * 0.5);

      const left = CHART_PADDING_LEFT + this.currentBarMouseOverTarget_ *
          barWidth - tooltipOffset;
      const top = Math.round((this.chartHeight_ - tooltipHeight) * 0.5);

      this.chartCtx_.sa