Hi Vito,
I am afraid that this feature is not currently supported by Essential Chart[4.2.0.37]. This feature was [ChartLegendReperesentationType.StraightLine] implemented internally in ChartControl.
Please take a look at the sample attached.
Let me know its fulfills your ChartInterior color setting requirement.
Thanks you for using Syncfusion Products.
Regards,
Rashidha
ChartInteriorSample.zip