Chart visualization

Preliminary documentation:

$ rosrun rosprolog rosprolog knowrob_vis
?- diagram_canvas.
?- add_diagram(id, title, type, xlabel, ylabel, [[[a,b],['1','2']],[[a,b],['1','2']],[[a,b],['1','2']]]).

This publishes the following message on the topic 'data_vis_msgs':

id: id
type: 0
title: title
xlabel: xlabel
ylabel: ylabel
values: 
  - 
    value1: ['a', 'b']
    value2: ['1', '2']
  - 
    value1: ['a', 'b']
    value2: ['1', '2']
  - 
    value1: ['a', 'b']
    value2: ['1', '2']

An example of the receiving side can be found here.