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Generates flow lines between discreet nodes for depicting spatial interaction data (e.g. migration).

Plugin needs two inputs: (i) text file storing node coordinates and (ii) text file storing
interaction matrix (a.k.a. flow data matrix)
In the first text file, each row corresponds for one node and it should contain one pair of
coordinates (x, y) delimited by white space. (Decimal separator for coordinates is point “.”)
In the second file, magnitudes of flows from the first node to last are given respectively.
Resulting text file stores a square interaction matrix whose values are white space delimited.
Each row corresponds to a “from” node and each column corresponds to a “to” node. The
order of magnitudes listed in this text file should conform to the order that is used in the first
file. (e.g. with reference to Figure 1 given in next page; number of outgoing bank interacts
from node 4 to 6 is 177, vice versa number of outgoing interacts from node 6 to 4 is 186.
Notice that matrix is not symmetric.)
Three types of flow lines can be generated by using this plugin: (i) two way, (ii) gross, (iii) net.
When two ways option is selected, both incoming and outgoing flow lines are generated.
When gross option is selected summation of interactions between a node pair is considered.