sadf: SVG: Always start id numbers at zero

sadf: SVG: Always start id numbers at zero

For actvities with multiple items, make sure that the graph id number
for the first item being displayed is zero ("gXX-0").
If this is not the case then the link from the table of contents will
not work. This could happen when "skipempty" option was used and the
first item in the list didn't have a graph because all its stats were
zero.

$ sadf -g -O showtoc,skipempty -- -A > output.svg

Signed-off-by: Sebastien GODARD <sysstat@users.noreply.github.com>