'Seaborn - ScatterPlot with Average Y-axis

I have written the below code in the seaborn scatterplot for the visual to create

sns.scatterplot(x="Year_Birth", y="NumStorePurchases", data=md)

I get the plot in the format below

enter image description here

I now need the average of the Y- axis and want to adjust the plot to look like the below picture

enter image description here

I have tried using the np.mean in the estimator but it has not worked out. Is there something that is being wrongly done? How this can be achieved?



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