Fix segfault on empty project, improve plotting

[paleolimbot]

No description provided.

[paleolimbot]

Fixes #59, fixes #52.

[codecov-commenter]

Codecov Report

Merging #61 (8ece413) into master (1a6ca9a) will increase coverage by 0.04%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #61      +/-   ##
==========================================
+ Coverage   97.21%   97.26%   +0.04%     
==========================================
  Files          31       31              
  Lines        2445     2482      +37     
==========================================
+ Hits         2377     2414      +37     
  Misses         68       68              

Impacted Files	Coverage Δ
src/geos-binary-atomic.c	94.54% <ø> (ø)
R/geos-plot.R	100.00% <100.00%> (ø)

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