Simplify plotting in `README` example

[navidcy]

I get the same movie.

[codecov[bot]]

Codecov Report

Patch and project coverage have no change.

Comparison is base (536459f) 52.78% compared to head (017cac7) 52.78%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #99 +/- ## ======================================= Coverage 52.78% 52.78% ======================================= Files 26 26 Lines 1023 1023 ======================================= Hits 540 540 Misses 483 483 ```

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[glwagner]

I guess more generally, as a rule one should never index with something like 1:grid.Nx.

[navidcy]

I think that something was messed up with the new gh-pages branch while this PR was open. Closing in favor of #100.