How to force two entities to point to the same lookup value

Please bear with me I've tried to make my question as terse as possible, whilst not appearing confusing (hopefully I've not failed in this regard).

I'm trying to store a Chart configuration in an Oracle database
(although I'm hoping that my choice of database will not lead to a
solution specific to it, since so far my database is not platform, or database specific).

One of these chart entities has two or more axes (one x and at least one y) related to it, and each axis relates to zero or more plot entities
(which store plot information such as plot type, line or columns/bars
etc). these plot entities relate to a specific item of data that can be plotted (and relates to a Unit table).

So far I have

relations-> Chart (1-3) Axis (0-many) Plot (Many-1) DataItem

(I'm assuming I've got this relationship right)

How can I now insert a relationship with a Unit entity so that I can ensure that Axis and Plot can refer to any unit in the unit relation *so long as its the same Unit*. I'd like to enforce at a relational level a limitation so that any number of plots can be configured for a specific axis so long as they are all of the same unit. For instance income and outgoings can both be related to an axis since they share the same unit of currency. But weight and income cannot be plots that relate to one axis since there are two units here and an axis can refer to only one unit.