Use 'Time before present' in 'plot_mainland', fix #63

[richelbilderbeek]

Hi @joshwlambert,

Here is the plot with the reversed axis, as discussed at #63:

Enjoy, Richel

[codecov-commenter]

Codecov Report

Merging #67 (7cf1e51) into develop (c61194d) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff            @@
##           develop      #67   +/-   ##
========================================
  Coverage    98.98%   98.99%           
========================================
  Files           70       70           
  Lines         4649     4657    +8     
========================================
+ Hits          4602     4610    +8     
  Misses          47       47           

Impacted Files	Coverage Δ
R/plot_mainland.R	100.00% <100.00%> (ø)

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