SVG: Fix graphs for swap space utilization statistics

SVG: Fix graphs for swap space utilization statistics

Commit 8b71682 added a new metric to sar's memory report (available free
memory). This new metric caused a shift in the array containing data to
be drawn that wasn't properly taken into account (see commit f90adb6).
As a consequence graphs for swap space utilisation statistics (those
corresponding to sar -S output) were wrong.
This patch fixes the problem by using the right position in data array.

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