'Plotting a reference line over facet plots

I need to scatter plot data with its own line by type with a comparison to a reference line for each facet. I am wrestling with getting the line equation y=8x+10 to plot on each facet plot.

    import pandas as pd
    import seaborn as sns

    sns.lmplot(x="18O‰ VSMOW", y="D‰ VSMOW", hue="Type",
        col="Type", col_wrap=2, data=df)

My goal is to enable easy comparison of each Type to a known general relationship. Below, I drew in what I would like on the top two plots:

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'Plotting a reference line over facet plots

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