如何在Pine脚本中获取当前条的前一个主要轴心？

//@version=4
study("获取前一个主要轴心", overlay=true)

// 定义一个变量来保存前一个主要轴心的索引
var int prevPivotIndex = na

// 判断当前条是否为主要轴心
isPivot = high > high[1] and high > high[2] and high > high[3] and high > high[4] and
           high > high[5] and high > high[6] and high > high[7] and high > high[8] and
           high > high[9] and high > high[10] and high > high[11] and high > high[12] and
           high > high[13] and high > high[14] and high > high[15] and high > high[16] and
           high > high[17] and high > high[18] and high > high[19] and high > high[20]

// 如果当前条是主要轴心，则更新prevPivotIndex变量
if isPivot
    prevPivotIndex := bar_index

// 获取前一个主要轴心的索引
prevPivotIndex := ta.valuewhen(isPivot, bar_index, 1)

// 输出前一个主要轴心的索引
plot(prevPivotIndex, title="前一个主要轴心的索引", color=color.blue, linewidth=2)