If no title is set for yAxis, it behaves correctly, there is an horizontal line

But if we set a title, the line becomes diagonal:

To reproduce,

Go on https://www.chartjs.org/chartjs-plugin-annotation/master/guide/types/line.html#configuration and add in the options:

scales: {
    yAxis: {
        title: {
            display: true,
            text: "My First Dataset"
        }
    }
},