attribute may not be variable

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Describe the bug

procs = GET process
        FROM file:///tmp/lab101.json
        WHERE parent_ref.name = 'svchost.exe'
        START 2021-04-03T00:00:00Z STOP 2021-04-03T02:00:00Z
        
procs_grps = GROUP procs BY binary_ref.name WITH COUNT(pid) AS number_of_procs

APPLY python://attribute-plot ON procs_grps WITH XPARAM=binary_ref.name, YPARAM=number_of_procs

error:

[ERROR] KestrelSyntaxError: invalid token "'binary_ref.name'" at line 6 column 29, expects one of ['BIN', 'ATTRIBUTE']
rewrite the failed statement.

Kestrel version: v1.5.1

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need to document it that to use 'binary_ref.name' to tell Kestrel treat it as a string.

This works:

procs = GET process
        FROM file:///tmp/lab101.json
        WHERE parent_ref.name = 'svchost.exe'
        START 2021-04-03T00:00:00Z STOP 2021-04-03T02:00:00Z
        
procs_grps = GROUP procs BY binary_ref.name WITH COUNT(pid) AS number_of_procs

# need to single quote binary_ref.name so Kestrel will not treat binary_ref as a variable
APPLY python://attribute-plot ON procs_grps WITH XPARAM='binary_ref.name', YPARAM=number_of_procs