new section "Multiple variables visualisation"

docbook/users-guide/Trees.md

docbook/users-guide/para11.png

docbook/users-guide/pictures/bp1.png

docbook/users-guide/pictures/para1.png

docbook/users-guide/pictures/para10.png

docbook/users-guide/pictures/para11.png

docbook/users-guide/pictures/para12.png

docbook/users-guide/pictures/para13.png

docbook/users-guide/pictures/para14.png

docbook/users-guide/pictures/para2.png

docbook/users-guide/pictures/para3.png

docbook/users-guide/pictures/para4.png

docbook/users-guide/pictures/para5.png

docbook/users-guide/pictures/para6.png

docbook/users-guide/pictures/para7.png

docbook/users-guide/pictures/para8.png

docbook/users-guide/pictures/para9.png

docbook/users-guide/pictures/spider1.png

docbook/users-guide/pictures/spider2.png