Use nice formatting of values

For the "usual" use of bar graphs, say for percentages, this
doesn't matter because the range 0-100 is readable anyway.
But for silly graphs like free memory you want
to have a formatted value rather than 'g' style floats
(2.1+e07 vs 21000000 -- neither is readable as 21M but
for that we would need to have a formatter option
"whatever metric prefix fits")