HPCC-11066 Ensure graph overview is not blank

Older graph controls can display a blank graph at level 1 (mostly for simple
graphs).

Fixes HPCC-11066

Signed-off-by: Gordon Smith <gordon.smith@lexisnexis.com>
Merge pull request #5577 from GordonSmith/HPCC-11066

HPCC-11066 Ensure graph overview is not blank

Reviewed-By: Miguel Vazquez <miguel.vazquez@lexisnexis.com>
Reviewed-By: Richard Chapman <rchapman@hpccsystems.com>