Add plotgrid and fix grid

[codecov[bot]]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.01% :tada:

Comparison is base (f394e8f) 98.32% compared to head (97b63f4) 98.33%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #38 +/- ## ========================================== + Coverage 98.32% 98.33% +0.01% ========================================== Files 5 5 Lines 537 541 +4 ========================================== + Hits 528 532 +4 Misses 9 9 ``` | [Files Changed](https://app.codecov.io/gh/JuliaApproximation/PiecewiseOrthogonalPolynomials.jl/pull/38?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaApproximation) | Coverage Δ | | |---|---|---| | [src/PiecewiseOrthogonalPolynomials.jl](https://app.codecov.io/gh/JuliaApproximation/PiecewiseOrthogonalPolynomials.jl/pull/38?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaApproximation#diff-c3JjL1BpZWNld2lzZU9ydGhvZ29uYWxQb2x5bm9taWFscy5qbA==) | `100.00% <ø> (ø)` | | | [src/arrowhead.jl](https://app.codecov.io/gh/JuliaApproximation/PiecewiseOrthogonalPolynomials.jl/pull/38?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaApproximation#diff-c3JjL2Fycm93aGVhZC5qbA==) | `98.64% <100.00%> (+<0.01%)` | :arrow_up: | | [src/continuouspolynomial.jl](https://app.codecov.io/gh/JuliaApproximation/PiecewiseOrthogonalPolynomials.jl/pull/38?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaApproximation#diff-c3JjL2NvbnRpbnVvdXNwb2x5bm9taWFsLmps) | `97.39% <100.00%> (-0.21%)` | :arrow_down: | | [src/dirichlet.jl](https://app.codecov.io/gh/JuliaApproximation/PiecewiseOrthogonalPolynomials.jl/pull/38?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaApproximation#diff-c3JjL2RpcmljaGxldC5qbA==) | `96.96% <100.00%> (+0.67%)` | :arrow_up: | | [src/piecewisepolynomial.jl](https://app.codecov.io/gh/JuliaApproximation/PiecewiseOrthogonalPolynomials.jl/pull/38?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaApproximation#diff-c3JjL3BpZWNld2lzZXBvbHlub21pYWwuamw=) | `100.00% <100.00%> (ø)` | |

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