Scatter chart

Data sets with scattered points, like the "latency vs. time" view, are
now plotted in a scatter chart. The type of data set is defined by the
string "scatter" in the output id.

Each point of the data set can be displayed individually (scatter) or
connected by a line.

This pull request replaces
eclipse-cdt-cloud#399.

Contributes towards fixing eclipse-cdt-cloud#45.

Signed-off-by: Rodrigo Pinto <rodrigo.pinto@calian.ca>