Chartjs-I want to draw a horizontal line

Works fine when I test it. Do you import the plugin correctly, something like
<script src="https://cdnjs.cloudflare.com/ajax/libs/chartjs-plugin-annotation/0.5.7/chartjs-plugin-annotation.min.js"></script> ?

Here is my test-jsbin where it works exactly like it’s supposed to with your code (I only changed the value).