I posted my answer in your support ticket. For convenience I am pasting my reply here too, so the other users who are looking for a similar functionality can read it:
Generally speaking, setting a simple line to a particular value is not supported out of the box in Kendo UI Chart. As another workaround I can suggest (it seems you have tested the plotBands approach) to combine your chart with a line chart that has a constant value repeated across.
the Telerik team
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