Fix error with plotting block structure (#66)

[maltezfaria]

Fixes #66

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 3 lines in your changes missing coverage. Please review.

Project coverage is 75.59%. Comparing base (fbebdb2) to head (cf3a3bf).

Files	Patch %	Lines
src/hmatrix.jl	0.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #67      +/-   ##
==========================================
- Coverage   75.64%   75.59%   -0.06%     
==========================================
  Files          14       14              
  Lines        1466     1467       +1     
==========================================
  Hits         1109     1109              
- Misses        357      358       +1     

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