PieChart: touched section index is incorrect if some values are zero

PieChart: touched section index is incorrect if some values are zero. For example,

` values: 0, 0, 100, 200, 300.`

when touching the 3rd value 100, touched index is 0 that should be 2.

Start at the PieChart touch-handling entry point and reproduce the issue with values 0, 0, 100, 200, 300. Trace how the touched section is mapped to the input values, then verify that touching 100 reports index 2 rather than 0. Add or update regression coverage if the project provides a relevant test location.

Written by the indexing model from the issue text.