sadf: SVG: Display graphs for swap utilization in packed mode

sadf: SVG: Display graphs for swap utilization in packed mode

When both memory and swap utilization were selected, graphs for swap
utilization weren't displayed by sadf if option "packed" had been
entered, e.g.:

$ sadf -g -O packed -- -rS > output.svg

This was because SVG canvas height was not big enough. Indeed,
sadf assumed that each activity only needed one row in "packed" mode,
which is wrong for A_MEMORY activity (which requires two rows: One for
memory utilization and one for swap utilization).
This patch fixes the problem.

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