fix subset errors for bar plot

[raprasad]

The following sequence is causing errors in the JS console (Chrome/Mac)

1. Start the app (e.g. opens fearonLaitin)
2. Subset a variable bar plot variable; choose a 5-10 countries from country
3. Ajax call succeeds but errors in redrawing similar to below:

apparent cause

I'm getting similar results for the older TwoRavens in production.

The subset is too narrow and preprocess marks all the variables as valid = 0

fix

Some type of message is needed for the user.

• communicate that the subset is too narrow and doesn't contain enough data for calculations?
• needs a message in the JSON coming from Rook

[raprasad]

(moved to main issue)

[raprasad]

(moved to main issue)