How to add heatmap to quantmod::chart_Series?

I would like to plot heatmap(s) below quantmod::chart_Series(). How to add the below heatmap to chart_Series (or xts::plot.xts):

library(quantmod)

# Get data fro symbol from Google Finance
symbol <- "SPY"
src <- "google"
from <- "2017-01-01"
symbolData <- getSymbols(symbol, src=src, from=from, auto.assign=FALSE)

# Calculate simple returns
symbolData.ret <- ROC(Cl(symbolData), type="discrete")

# Calculate lagged autocorrelations (Pearson correlation for each value of lag)
nLags <- 100
averageLength <- 3
symbolData.laggedAutocorr <- matrix(0, nLags, NROW(symbolData.ret))
for (lag in 2: nLags) {
    # Set the average length as M
    if (averageLength == 0) M <- lag
    else M <- averageLength
    symbolData.laggedAutocorr[lag, ] <- runCor(symbolData.ret, lag(symbolData.ret, lag), M)
}
symbolData.laggedAutocorr[is.na(symbolData.laggedAutocorr)] <- 0
symbolData.laggedAutocorr.xts <- reclass(t(symbolData.laggedAutocorr), symbolData)ž
subset <- "2017"
chart_Series(symbolData, name=symbol, subset=subset)

# Use transposed symbolData.laggedAutocorr for plot so you have data aligned to symbolData
# How to add the below heatmap to chart_Series?
heatmap(symbolData.laggedAutocorr.xts, Rowv = NA, Colv = NA, na.rm = TRUE, labCol = "")

add_Heatmap <- function(heatmapdata, ...) {
    lenv <- new.env()
    lenv$plot_ta <- function(x, heatmapdata, ...) {
        # fill in body of low level plot calls here
        # use a switch based on type of TA to draw: bands, bars, lines, dots...
        xsubset <- x$Env$xsubset
        #heatmapdata <- heatmapdata[subset] # TODO: Something is wrong if I have a subset here
        heatmap(heatmapdata, Rowv=NA, Colv=NA, na.rm=TRUE, labCol="")
        #image(1:NROW(heatmapdata), 1:NCOL(heatmapdata), coredata(heatmapdata), axes=FALSE)
    }
    mapply(function(name, value) {assign(name,value,envir=lenv)},
            names(list(heatmapdata=heatmapdata,...)),
            list(heatmapdata=heatmapdata,...))
    exp <- parse(text=gsub("list","plot_ta",
                    as.expression(substitute(list(x=current.chob(),
                                            heatmapdata=heatmapdata,
                                            ...)))), srcfile=NULL)
    chob <- current.chob()
    chob$add_frame(ylim=c(0, 0.3), asp=0.3)  # need to have a value set for ylim
    chob$next_frame()
    chob$replot(exp,env=c(lenv, chob$Env),expr=TRUE)

    chob
}

chart_Series(symbolData)
add_Heatmap(symbolData.laggedAutocorr.xts)

The above almost works... The issue is that the heatmap or image is plotted over the main part of chart_Series instead below of it. What to do in order for it to plot correctly?