SVG: Handle files with orphan samples properly

SVG: Handle files with orphan samples properly

If a file has only orphan samples then display "No data" indicating that
the graph cannot be drawn.
An orphan sample is something like:

sadc data        <-- LINUX RESTART
sadc data 1 1    <-- Create an orphan sample
sadc data        <-- LINUX RESTART

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