Sadie found where the CDAWeb source doesn't slice properly with
vap+cdaweb:ds=MMS1_FGM_SRVY_L2&filter=mms&id=mms1_fgm_b_gse_srvy_l2_clean[:,3]&timerange=2017-11-28
but does with
https://cdaweb.gsfc.nasa.gov/sp_phys/data/mms/mms1/fgm/srvy/l2/2017/11/mms1_fgm_srvy_l2_20171128_v5.115.0.cdf?mms1_fgm_b_gse_srvy_l2[:,3]
Here's a screenshot of the two plots, bottom one slicing properly. Interesting the bottom one doesn't offer the slice in the GUI, but works, while the top one does offer it but doesn't do the slice.
This was to plot just the component "bt".
This is now fixed, checking for [:,i] after the virtual variable is loaded. Note non-virtual variables did not have this problem.