Add precipitation trendline at the top of a heatmap

[heatheranneshaw-13]

I have used the wtr.heat.map function to create the following heat map

I'm now wanting to add a precipitation trendline at the top of the heat map. Does anyone know how I can do this?

This is my code so far:

TalusSouthHeatMap <- wtr.heat.map(TalusSouthdf, zlim = c(0, 20),
plot.title = title(
main = "Talus South Water Temp (C)",
xlab = "Date",
ylab = "Depth (m)"))