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I guess you found a solution in the meanwhile but maybe I'll provide mine just in case.
I do this by creating a custom view first in NMC, containing the specific instance (in this case my single disk = filer:0a.19:disk:total_transfers) on which I want to collect metrics. I name this view total_transfers_single_disk.
Then I call this view from dfm CLI:
dfm perf data retrieve -V total_transfers_single_disk -M January > output-file.txt
The resulting file includes only the counter for my single disk. I am using DFM 5.1
About the mean, I would parse the dfm perf data retrieve output with powershell and possibly inject and format with excel cmdlet.