ENH: added single click selection in chartXY
Feature requested in https://discourse.slicer.org/t/plotting-select-point-on-click/3568 Necessary for https://github.com/Slicer/Slicer/pull/994
Edited by Davide Punzo
Feature requested in https://discourse.slicer.org/t/plotting-select-point-on-click/3568 Necessary for https://github.com/Slicer/Slicer/pull/994