add transparency to Area Available Line chart

[danangmassandy]

This is for #2055. Preview:

[lgkgh]

Jenkins Build Details:

Build status: SUCCESS Build duration: 14 min

[github-actions[bot]]

☂️ Python Coverage

current status: ✅

Overall Coverage

Lines	Covered	Coverage	Threshold	Status
16986	16485	97%	0%	🟢

New Files

No new covered files...

Modified Files

No covered modified files...

updated for commit: 6c0d8a6 by action🐍

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 97.25%. Comparing base (00160eb) to head (6c0d8a6).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #2068 +/- ## ========================================== - Coverage 97.25% 97.25% -0.01% ========================================== Files 374 374 Lines 16788 16788 ========================================== - Hits 16328 16327 -1 - Misses 460 461 +1 ```

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[dimasciput]

@danangmassandy The chart is difficult to see. Perhaps you could add more transparency to the area available for each species?