Feature Request: Single Stat specifying value to display

Right now the Single Stat graph type only displays the `_value` of the formatted CSV that's been passed in, but there are situations when a user will want to specify the value that of the CSV that's being passed in for them to display that isn't necessarily the `_value` column

Start by locating the Single Stat graph type and tracing how it reads the formatted CSV, especially the hard-coded `_value` column. Define how a caller specifies an alternate value column, then verify that the Single Stat displays the selected CSV value instead of always using `_value`.

Written by the indexing model from the issue text.