I think those settings might make a single-value visualization (lol, which has two values on it!) compare the most recent hour with the previous hour? Your results would seem to sort of possibly fit with that.
So try this:
index="network" sourcetype="network:proxy" earliest=-2w@w
| timechart span=1w eval(sum(bytes)/1024/1024) as totMBs
Keep in mind that compares the last full week's data to this week so far, which could be only the first days's worth of data from this week.
Better, perhaps, may be to press timewrap into service? The Usage and Basic Example section is pretty close to what you need, just that you have a slightly different base search.
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