When we resume work on Build an Atom in phetsims/build-an-atom#156 we will decide whether or not PeriodicTableCellIO even needs x/y values and, if it does, we could add them to the state through toStateObject or nested properties. I'll close this issue and open a new one for PeriodicTableCellIO.js.
From https://github.com/phetsims/phet-io/issues/1214#issuecomment-411447622
When we resume work on Build an Atom in phetsims/build-an-atom#156 we will decide whether or not PeriodicTableCellIO even needs x/y values and, if it does, we could add them to the state through toStateObject or nested properties. I'll close this issue and open a new one for PeriodicTableCellIO.js.