Fixed custom_plot

[pariterre]

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This change is 

[codecov]

Codecov Report

Attention: 9 lines in your changes are missing coverage. Please review.

Files	Coverage Δ
...ioptim/examples/getting_started/custom_plotting.py	82.14% <100.00%> (ø)
bioptim/limits/path_conditions.py	87.47% <100.00%> (ø)
bioptim/gui/plot.py	69.10% <77.77%> (-0.14%)	⬇️
bioptim/optimization/optimal_control_program.py	85.88% <72.00%> (+1.62%)	⬆️

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