Hi Karim,
There are a few tutorials that teach you how to customize or how to build charts:
Chart Customization: Chart Customizations in Mendix - YouTube
Build Charts: HOW TO BUILD CHARTS | Mendix 10 - YouTube
If you prefer the text version: Chart Configuration | Mendix Documentation
Through these customizations it is possible to change the tooltip style.
If you are using Plotly, you can simply use "hoverlabel" property in the advance tab in the widget properties
If this answer helped you, please mark as accept :)
Best Regards,
Ricardo Pereira
You could check out these options. Mendix documentation is referring to plotly for more detailed information about layout and config options as well.