Chartjs-Annotation library for chartjs

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This is happening because you are setting the labels as an array of numbers and the annotation plugin, when the scale is a category one, use the index of the array when the value is set as number. In fact, 7.5 is the index between values 24 and 25.

To have the line at value 7.5 you need to set the annotation value to 2.875.

Why? Because the value 7.5 is between index 2 and 3 and the difference of the values at index 2 (value 4) and 3 (value 8) is 4. Therefore the value is:

((7.5 - 4) / 4) + 2 = 2.875
        |    |    |
        |    |    +--> base index 
        |    +--> difference between index 3 and 2
        +--> value at 2

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