When a formula object is selected (in any LO application), its context menu, as well as Edit-Object menu, contain "Save copy as..." item, which allows saving to ODF Formula (.odf) and MathML 2.0 (.mml). The proposal is to make the same for charts, which have their own ODF Chart format - ODC (you may save a chart if you enter edit mode on it by double-clicking, and use File-Save As... menu).
Good idea -> NEW
Obsolete now with export to ODC removed in commit https://git.libreoffice.org/core/+/d543e8dfd3d8ae718e29304a155e1cf70d08325d.