Remove default projectors

Having default accessors in our projectors can lead into very annoying circumstances in our codebase where we have to make the assumption between the accessor being invalid or valid rather than the easier-to deal-with null or valid.

Thus, the "x", "y", "value" default accessors have been removed from xy-plots and gridplots, respectively.

Also, for StackedArea plots, if the projectors are not ready, then stackOffsets will not be updated until all the projectors are ready.

QE: While the level of complexity of this PR is not too high, this had a high surface-area of touch.  A regression pass and testing #1335 would be needed, I think.