plotter: EventPlot: avoid changes on input data by operating on a copy

Created by: mdigiorgio

_get_lanes() does some changes to the input data that are necessary for the Javascript code to work properly. However, these changes permanently affect the input data. In particular, "lane" names are converted to integers.

To avoid this, let's create an internal copy of the input data and always operate on that.

Signed-off-by: Michele Di Giorgio michele.digiorgio@arm.com