scatterplot.js: fix a bug with update radius
requested to merge feature-c/4374-update-radius-ne-renvoye-jamais-de-taille-inferieur-a-2px into dev
-update radius was never able to return a value < 2px for an already existing clone -add watir tests
Closes #4374 (closed) also seen in #4014 (closed) #4015