Horizontal lines are great for showing spots where something happened on the y-axis.
Matplotlib provides the axes.hlines() method to plot horizontal lines.
Here is the example
And here is the result.
Here you are! You now know how to plot a horizontal line with Matplotlib.
More on plots
If you want to know more about how to add labels, plot different types of plots, etc... check out the other articles I wrote on the topic, just here :