Add better handling of fields

[Daniel Marsh-Patrick]

In #9 we had to change the data view mappings and relax the rules a bit. Subsequently, this will still try and render the chart for any combination of fields. Need to allow the following combinations only:

• Category, Sampling and Value
• Sampling and Value

All others should be rejected. We should also provide some help in the viewport if this occurs so that the user can understand what they need to do to get started.

[Daniel Marsh-Patrick]

#28: Reworked DOM to allow additional of an 'info' box if required fields are missing, and prevent the chart render from going ahead. This should help us when we get to #33 also. Resolves #28.

→ <<cset 64b604fa739e>>