<｜begin▁of▁sentence｜>### Instruction:
Given a graph which you can read from 'graph_data.gml', can you calculate the hierarchy of its cohesive blocks using the 'hierarchy' method from the igraph library's CohesiveBlocks class and plot this hierarchical tree?
### Response:
from igraph import Graph

# Read a graph from a file
g = Graph.Read_GML('graph_data.gml')

# Calculate the cohesive blocks of the graph
cohesive_blocks = g.cohesive_blocks()

# Get the hierarchy of cohesive blocks
hierarchy_graph = cohesive_blocks.hierarchy()

# Plot the hierarchy graph
hierarchy_graph.vs['label'] = range(len(hierarchy_graph.vs))
ig.plot(hierarchy_graph, bbox=(300, 300), margin=20)
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