Others have suggested taking a keyword you rank #1 or #2 for, seeing what your daily traffic is from that keyword and comparing other search terms to that... so you can see the relative difference and estimate a number.
But a pain, I agree
I believe those bar a relative to other country, not the search volume...
let say keyword A and keyword B 's search volume breakdown for different country are the same
US: 50%
UK: 30%
canada: 10%
Aussie: 5%
and keyword A is being search for 200k times/month and keyword B is being search for 1k times/ per month
the graph will be the same.
just like the graph on top, it is relative to % change between month instead of search volume