Hi Luca,
Greetings from Syncfusion. We have analyzed your scenario at our end and based on the provided screenshot we suspect that you are using CategoryAxis as the x-axis in your chart sample and due to this only when using the point to pixel method to get the offset/pixel location where the axes cross each other, you got the offset of the first point in the series. So, we kindly request you to use the NumericAxis for the x-axis or set the labelPlacement property of CategoryAxis as LabelPlacement.onTicks and then try getting the pixel location of the (0,0) point value in the chart using the pointToPixel method.
Please check with the above solution and revert us if you require further assistance on this.
Regards,
Sriram Kiran