//@version=6
indicator("Workshop 14 Pair Distance", overlay=false)
other = input.symbol("NASDAQ:PEP", "Second leg")
beta = input.float(1.0, "Fixed price coefficient",
    minval=0.0)
length = input.int(60, "Reference chart bars", minval=2)
[peer, peerTime] = request.security(other,
    timeframe.period, [close, time_close],
    gaps=barmerge.gaps_on, lookahead=barmerge.lookahead_off)
aligned = not na(peer) and peerTime == time_close
difference = aligned ? close - beta * peer : na
center = ta.sma(difference, length)[1]
deviation = ta.stdev(difference, length, true)[1]
count = math.sum(aligned ? 1.0 : 0.0, length)[1]
ready = aligned and count == length and deviation > 0
z = ready ? (difference - center) / deviation : na
plot(z, "Spread distance", color=color.blue,
    style=plot.style_linebr)
hline(0, "Prior center", color=color.gray)
hline(2, "Upper observation", color=color.gray)
hline(-2, "Lower observation", color=color.gray)