@petridispa,
Can you kindly create a JSFiddle reproducing the issue you are facing and share it with us along with sample data so that we can look into your code / chart-options being used, understand the scenario better and help you out.
Also take a look at this documentation page for a step-by-step tutorial on rendering charts with data fetched from JSON API using AJAX.
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Adithya Menon
Team CanvasJS