SVG: Define a max number of horizontal lines for the background grid

SVG: Define a max number of horizontal lines for the background grid

When a graph for percentage values is displayed, a background grid with
horizontal lines for 25%, 50%, 75% and 100% is drawn.
This works well except when sadf gets a bogus value for the maximum
value reached by the metric. For example, if sadf thinks that the max
value is 123456789% then a high number of horizontal lines will be
drawn, resulting in a huge SVG file that can weigh hundreds of megabytes
or more (NB: There are metrics which are percentage values that can go
above 100%, e.g., %commit).
This patch fixes that by setting a limit for the number of horizontal
lines that can be drawn.

See issue #150.

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